Inline image
An image displayed within the body of an HTML email, either embedded in the message as a MIME part referenced by cid: or loaded from a web address.
What it means
There are two ways to show an image in an HTML email. Embedding puts the image in the message as a part of a multipart/related structure with a Content-ID header, and the HTML refers to it with a cid: URL (RFC 2392); it displays without a network fetch but makes the message larger. Linking uses an ordinary https URL to an image on a web server; the message stays small, but the image appears only when the mail program fetches it.
Linked images are subject to image blocking: classic Outlook blocks automatic picture downloads by default, and Gmail hides images when it considers a sender or message suspicious. Linked images are also what open tracking depends on. Base64 data: URLs inside the HTML are a third option, supported unevenly across mail programs.
Related terms
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 2387: The MIME Multipart/Related Content-typerfc-editor.org
- RFC 2392: Content-ID and Message-ID Uniform Resource Locatorsrfc-editor.org
- Microsoft Support: Block or unblock automatic picture downloads in classic Outlooksupport.microsoft.com
- Google, Gmail Help: Turn images on or off in Gmailsupport.google.com
- Can I email: Base 64 image formatcaniemail.com
All 432 terms in the glossaryThe wiki: blocklists and bounce codes