What are inline vs. attached images?

Inline images display directly in the email body, like logos or banners embedded in your message. Attached images show up as separate files the reader has to download to view.

You see inline images all the time in newsletters and promotional emails. The header graphic, product photos, social icons, they're all inline. Attached images are what you'd send in a personal email when sharing vacation photos or a document scan. They appear as separate items at the bottom of the message (or in a paperclip list), not woven into the content itself.

The technical difference: inline images use Content-ID (CID) references in the email's multipart structure, which tells the email client "this image is part of the body, display it here." Attached images are just files that happen to be included with the email.

But here's the practical concern: Gmail and most modern email clients block images by default until the reader clicks "Show images." That means your inline images might not display at all on first open. (Of course, if the reader trusts you, or if they've already shown images from your domain before, they'll load automatically.) Attached images don't have this problem because they're not trying to render in the body, but that's a trade-off. Nobody wants to download five files just to see your product lineup.

For marketing emails, inline is almost always the right choice. You want the visual experience to be immediate (once images load). For transactional emails like receipts or password resets, skip the inline images entirely unless they're genuinely necessary. Text-based transactional emails are faster, more accessible, and never break when images don't load.

One more thing: some spam filters flag emails with large attached images as suspicious, especially if the attachment is the only content in the email (like those old-school "image-only spam" campaigns). Inline images embedded properly in HTML don't trigger that concern. Worth checking your image setup with our Source Analyzer if you're seeing delivery issues and you're heavy on visuals.