Images

Images in a Sigil signature are attached to the message inline rather than loaded from a web server. The add-in attaches each one and the template references it by name.

Not every image in a signature is an upload. A QR code and a sender’s own profile photo are generated per person when the signature is served, and the designer’s social icons are drawn on request from artwork Sigil ships. None of the three appears in the image library and none of them counts against it. See per-user images and social icons.

Why not a hosted image

Outlook blocks external images by default. A signature using <img src="https://example.com/logo.png"> renders as a broken image placeholder for most recipients until they choose to download pictures, which most people never do.

Inline attachment sidesteps that entirely. The image travels with the message and displays immediately.

Uploading

Upload images in the Images view of the portal. Each one is stored against your organisation and is available to any template in your library.

Uploading needs the admin or editor role.

1. Open Images in the portal sidebar.
2. Choose Upload image.
3. Pick a PNG or JPG file of up to 1 MB.

The new row shows the stored name and the cid: reference to use in a template. The designer’s Image block has its own Upload button, which adds to the same library.

Uploads are recorded in the change log along with who uploaded them.

A logo read from your website when you start a template from it is stored here too, in the same way.

The name an image ends up with

The name is also the cid: reference a template uses, so it has to survive being written into HTML. Sigil normalises it on upload rather than refusing the file: spaces become dashes, characters outside letters, digits, dots, dashes and underscores are dropped, and leading and trailing separators are trimmed. A file called Acme Logo (final).png is stored as Acme-Logo-final.png. The Images view shows the name that was actually used, which is the one to reference.

Names beginning sigil- are reserved and refused. Sigil uses that prefix for the QR codes and profile photos it generates for each sender, and an upload under it would never be shown.

Uploading a file whose own name is already in the library replaces that image in place. That is how a logo is updated: every template referencing it picks up the new bytes on the next compose, with nothing to republish.

Replacement needs the file name to arrive as the stored one, though. Where normalisation had to change the name and the result is already taken, Sigil stores a numbered variant rather than overwriting. Landing on an existing name after being rewritten is a coincidence rather than an intention, and overwriting the logo every live signature references would be an expensive way to discover that. The practical consequence is that a logo you want to replace should be re-uploaded under exactly the name shown in the Images view, so Acme-Logo.png rather than the Acme Logo.png still sitting on your desktop.

To replace an image:

1. Open Images in the portal sidebar and note the Name shown for the image.
2. Give the new file exactly that name on your computer.
3. Choose Upload image and pick the file.

Every template referencing the image picks up the new file on its next compose, with nothing to republish.

Seeing what you have

The Images view shows a thumbnail of every uploaded image alongside its name and the cid: reference to use in a template. Selecting a thumbnail opens the image at full size with its dimensions in pixels and its file size.

The file size is the number to look at. It is the one property of an image that affects every message anyone sends, and it is invisible from the file name.

Referencing an image

In the HTML editor, reference an uploaded image by name with a cid: URL:

<img src="cid:logo.png" width="140" alt="Acme">

In the designer, add an Image block and pick the image. The compiler emits the same reference.

1. Drag Image from the Blocks rail onto the canvas.
2. In the inspector, under Image, pick the image from the list, or choose Upload to add one from your computer.
3. Set the width and height in pixels, and the Alt text.

Always set an explicit width in pixels. Outlook does not reliably scale images without one, and a logo exported at 2x will render at twice the size you intended.

Images on a phone screen

A phone can narrow a signature to fit its screen, and an image with a fixed width and height does not always survive that. Outlook for Android shrinks the width but keeps the height as set, so a wide logo comes out squashed.

The designer guards against this for you. Image and Photo blocks carry max-width:100%;height:auto in their inline style beside the width and height attributes, so a client that narrows the picture keeps it in proportion. Classic Outlook for Windows ignores that style and sizes the image by its attributes, exactly as before. QR code and social icon blocks do not carry it.

The designer’s HTML is built when you publish, so a design last published on or before 15 September 2026 may still have the old markup. Publish it again to pick this up.

The HTML editor sends your markup as written, so add the same style yourself:

<img src="cid:logo.png" width="140" height="40" alt="Acme" style="display:block;border:0;max-width:100%;height:auto;">

Formats

Use PNG or JPG. They are the only formats an upload accepts, and the check reads the file itself rather than trusting its extension, so a GIF renamed to .png is refused.

SVG is not supported. Outlook does not render it, and a signature containing one will show nothing where the image should be. Uploading an SVG to the image library is refused with “SVG is not supported in mail signatures; use PNG or JPG”.

PNG is the usual choice for logos, because it keeps edges crisp and supports transparency. JPG suits photographs, such as a headshot.

Size

Every attached image adds to the size of every message anyone sends. A 400KB logo on a mailbox sending a hundred messages a day is 40MB of avoidable traffic per person per day.

Export at the size you will display, at roughly twice that in pixels for sharp rendering on high-density screens, and compress. A typical signature logo should be a handful of kilobytes.

An image larger than 1 MB is refused at upload. That is a ceiling for the unusual case, such as a photograph, rather than a size to aim for.

The 30,000 character limit on a rendered signature applies to the HTML rather than to the attachments, so images do not count against it. See Outlook constraints.

How much the library holds

An organisation’s library holds up to 250 images and 50 MB in total. The Images view shows both as meters, which turn amber at 80% and red when the library is full. New uploads are then refused until something is deleted.

Uploading under the name of an image already in the library replaces it. The old file’s size is freed rather than added to, and the replacement does not take up another of the 250 places.

Every image in a signature travels inside the cached copy served for every mailbox, which is why the library has a ceiling at all. The limits sit far above what a real signature uses.

The pictures behind signature previews are not part of the library and do not count against either meter. Sigil makes them, and it deletes them with the version they show, so they are not yours to manage and not yours to pay for in shelf space.

Transparency

A signature sits on whatever background the recipient’s mail client uses, which may be white, off-white or dark. A logo on a transparent background adapts. A logo with a baked-in white rectangle looks like a white rectangle in dark mode.

Where a logo needs to work on both, a version with sufficient contrast against both is safer than relying on transparency alone. There is no dark variant slot on an image, and no client recolours one, so the single file has to work everywhere. See dark mode.

Alt text

Set alt on every image. It is what screen readers announce, and it is what appears in clients that suppress images entirely.

Keep it descriptive rather than decorative: alt="Acme" on a company logo, and an empty alt="" on a purely decorative divider so that assistive technology skips it.

Deleting an image

Deleting is blocked while anything still references the image. The refusal names what is holding it: each template whose live body uses it, each template whose unpublished draft uses it, and each banner that is the image.

Drafts count deliberately. A draft referencing a deleted image renders nothing the moment somebody publishes it, and the person publishing would have no reason to connect the two.

The check does not cover older versions in version history or templates in the bin. Rolling back to a version, or restoring a template, that used a deleted image brings it back without that image, so check before deleting an image you might want back.

Detach it from everything the message lists, then delete. Deletions are recorded in the change log, and the previous file is not retained, so keep your source files.

1. Open Images in the portal sidebar.
2. Choose Delete at the end of the image’s row.
3. Choose Delete in the confirmation.

An image something still uses is refused with a message listing what uses it.

Images and export

When you export a template, every referenced image travels with it inside the JSON bundle. Importing into another organisation recreates them.

If an imported image name collides with an existing one, the import creates a numbered variant and rewrites the cid: references to match, so nothing is overwritten.

Downloading a signature with images inlined

The download option, from both the portal and the “My signature” pane, produces a standalone HTML file with images inlined as data: URIs rather than as attachments. That makes the file self-contained and portable, which is what you want when pasting the signature somewhere else. See download a copy.