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An email signature image is a strange piece of design. It is small, it is seen constantly, and it has to survive three conditions that almost no other image faces — and the sizes follow from those conditions rather than from any official specification.
Many email clients do not load remote images until the reader clicks something. For a first email to someone who has never corresponded with you — exactly the situation where a signature matters most — there is a good chance your logo never appears at all.
Two consequences follow.
The signature must read as text. Name, title, company and phone number belong in actual text, not inside the image. This is the single most common and most damaging mistake: building the whole signature as one picture. When it is blocked, the recipient sees an empty rectangle and nothing else. The text is also unselectable, uncopyable, invisible to search, and unreadable by a screen reader even when it does load.
Alt text is not optional. A blocked image shows its alt text, so alt="Acme Ltd" still conveys the company name. An empty alt attribute conveys nothing.
Every modern screen packs two or more physical pixels into each layout pixel. An image whose file is exactly its display size looks soft on all of them.
The fix is standard: export at 2× and set the display width explicitly in the signature’s HTML. A logo meant to appear 180 pixels wide is exported at 360 and displayed at 180.
Setting that width matters. Without it the image renders at its full 360 pixels and your signature is twice the size you designed.
That 600-pixel ceiling is the width most email layouts are built around. Anything wider forces horizontal scrolling on a phone, which looks broken in a way nothing else in the message does.
Many clients now render messages on a dark background, and some invert colours automatically. A logo exported as a JPG carries its white background with it, so it appears as a bright white rectangle floating in a dark email.
Use a transparent PNG, and check how the logo looks against dark. A dark-coloured logo on transparency becomes nearly invisible; a logo with a thin white outline, or a version designed for dark backgrounds, survives both.
The simplest robust answer is a logo that works on any background — solid mid-tone colours, no near-white and no near-black elements.
A signature is attached to every message you send. At a thousand emails a year, a 300 KB signature is 300 MB of transfer and storage — in your sent folder and in every recipient’s inbox.
There is a second effect: on some systems a large signature counts toward the message size limit, so a near-limit attachment fails for reasons the sender cannot see.
Target under 20 KB per image and under 50 KB for the signature as a whole. A 360-pixel PNG logo of flat colours compresses to a few kilobytes; if yours does not, it is probably a photograph saved as PNG.
A linked image lives on a web server and the email references its URL. The message stays small, you can update the logo everywhere at once — and it is the version that gets blocked.
An embedded image travels inside the message. It always displays, but it adds its weight to every email, and some clients show it as an attachment, so your message arrives with a paperclip icon suggesting a document that is not there.
Linked is the better default for most people; just keep the URL stable, because a moved image breaks every signature ever sent.