Email Signature Image Sizes — Banner, Logo & Headshot

Nobody Publishes an Email Signature Size — Except Google, Once

Search this question and you will be handed confident numbers: a 600x200 banner, a 300x300 logo, a 100x100 headshot, keep it under 10KB. None of them are sourced, because there is nothing to source them to. Google, Microsoft and Apple all document how to put an image into a signature and none of them says how big it should be.Apple’s entire published guidance is to drag an image into the preview area. Microsoft tells you to right-click and use the Size tab, and names no target.

There is exactly one exception, and it is worth being precise about its scope. In Google Workspace, an administrator can append a footer to everyone’s outgoing mail, and for that image Google writes: if you add an image to the footer, we recommend an image that’s 70–100 pixels high by 300–400 pixels wide. The maximum size is 100 pixels high by 1000 pixels wide. That is an admin-appended org-wide footer — not your personal Gmail signature. But it is the only pixel dimension any email vendor has committed to in writing, it was written for an image in exactly this position, and it is a far better starting point than a number somebody made up.

So the honest recommendation is Google’s box, doubled for retina: a banner displayed at 400x100 and exportedat 800x200. The one place you must not double is Google’s 1000px ceiling — that is a stated maximum for the footer image file, so 1000x100 is where you stop, not a 2000px export.

Gmail vs Outlook vs Apple Mail

None of the three tells you what size to make the image. What they do have — and this is what actually decides whether your signature arrives intact — are three completely different sets of constraints, and each one breaks in its own way.

Gmail

• 10,000 characters, and your image counts toward it. This is the one hard limit in email signatures that a vendor actually states. Google: you can put up to 10,000 characters in your signature, and your image also counts toward the character limit. If you get an error, try to resize the image. That is as close as Google comes to giving you a size — an instruction to shrink it when it breaks.
• The image has to be publicly reachable. A signature image is referenced by URL, not shipped with the message. If you insert one from Drive, Google requires that you share the image publicly for it to appear in their signature. A private Drive file renders as a broken image for everyone but you.
• Base64 images do not render in Gmail. Inlining the image as a data URI to dodge hosting does not work — Gmail strips it, across web, iOS and Android. Host the file.
• Gmail proxies images through Google’s servers and caches them. Swapping the file at the same URL later may not update what past recipients see.

Outlook on Windows

• It renders through Microsoft Word. Microsoft documents this plainly — Outlook uses the HTML parsing and rendering engine from Word — and it is the root cause of everything below.
• max-width is not supported. Microsoft lists it as unsupported outright. The usual responsive-image trick of capping an image with max-width simply does nothing here, and in testing Outlook ignores CSS width on an <img> too. The HTML width attribute on the tag itself is the only lever that works.
• The high-DPI bug is real and Microsoft has a KB for it. On a display scaled past 100%, an image sent from Outlook appears larger in the recipient’s copy of the message than in the one you sent. Microsoft attributes it to the Word engine and fixes it with a registry value, DontUseScreenDpiOnOpen. This is why your signature logo arrives comically oversized and looks fine on your own screen.
• Animated GIFs show a single frame. Microsoft: only a static representation of the GIF image shows. An animated signature banner is a still image to a large share of your recipients.

Apple Mail

• Apple publishes nothing at all. The complete official guidance for a signature image is to drag it into the preview area. No dimensions, no formats, no file size.
• A dragged-in image is embedded, and embedding kills your sizing. Apple Mail ignores the HTML width attribute on embedded (CID) images — so an 800x200 file dragged into the signature editor renders at 800x200, not at the 400x100 you intended. This is the single most surprising behaviour on this page: the 2x export that makes your signature sharp everywhere else makes it twice as big here.
• The fix is to reference the image, not embed it. Paste signature HTML with an <img src="https://…" width="400"> pointing at a hosted file, exactly as Gmail requires anyway. One hosted, width-constrained image behaves correctly in all three clients; a dragged-in one does not.

Banner vs Logo vs Headshot

These are three different shapes with one shared constraint, and the constraint is height. A signature sits under every message you send, in a reading pane that is often only a few hundred pixels tall. Google’s footer guidance caps the image at 100px high, and that ceiling is the useful thing to borrow: whatever the shape, roughly 100px of vertical space is the whole budget.

• Banner — wide, and the only asset with room to be wide. Display at 400x100, export at 800x200. That is Google’s recommended box at 2x, and a clean 4:1. If you need the full width Google permits, 1000x100 is the documented maximum — and because it is a maximum, that one gets exported at 1x.
• Logo — square, 100x100 displayed, 200x200 exported. Same height budget as the banner, so the same 100px ceiling applies. A wordmark that is wider than it is tall should be treated as a small banner instead, not squeezed into a square.
• Headshot — the same square as the logo. This is the part every other guide gets wrong by making it a separate spec. A headshot and a logo occupy the same slot, under the same 100px height budget, so they are the same size: 200x200 exported, 100x100 displayed. Crop tight — a photo shown at 100px is a face, not a portrait, and anything below the shoulders is unrecognisable at that scale.
• Social icons — 48x48 exported, 24x24 displayed. Nothing is published here by anyone. 24px is simply the smallest a glyph can be and still read; doubling it keeps it crisp. Do not stack more than a handful — each one is another hosted request and another few hundred characters against Gmail’s 10,000.
• Do not use all three. A banner, a logo, and a headshot together is roughly 300px of vertical signature under a two-line email. Pick the one that does the work.

Sizing a profile photo for somewhere else at the same time? The LinkedIn image sizes guide covers the 400x400 profile photo and the 300x300 company logo — both are published specs, and both make good masters to downscale a signature headshot or logo from.

PNG or JPG — and Nothing Else

Format choice in email is not a matter of taste, because the modern formats you would reach for on the web do not render. PNG and JPG are the only two formats supported by every major client, and the gap between them and everything else is not close.

• PNG for logos, wordmarks, icons and anything with flat colour or text. PNG is lossless, so hard edges and type stay crisp. JPG compression puts visible artefacts around exactly those edges, which is what makes a JPG logo look slightly grubby at small sizes.
• JPG for headshots and photographs. A photo has no hard edges to protect and compresses far smaller as a JPG than as a PNG. This is the one place JPG wins outright.
• SVG is out. It does not render in Gmail, and it does not render in Outlook 2007 through 2016. A vector logo is the obvious thing to want in a signature and it is the one format you cannot use — export it to PNG.
• WebP is out. Unsupported in Outlook on Windows entirely, and converted to JPG by Gmail. No upside, real downside.
• Animated GIF renders as a still frame in Outlook. Design the first frame to work alone, or skip it.

One genuine trap: a transparent PNG logo can disappear in dark mode. Clients that invert or recolour the background behind an image do not touch the pixels inside it, so a dark logo on transparency ends up dark-on-dark. No vendor documents a fix. The reliable ones are to bake a background colour into the PNG, or to give a dark mark a light outline so it survives either background. The same problem bites email header images, where the sending platforms at least have opinions about it.

Frequently Asked Questions