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.

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.

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

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 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.

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.

Formats

Use PNG or JPG.

SVG is not supported. Outlook does not render it, and a signature containing one will show nothing where the image should be.

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.

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.

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.

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.

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.