Email signature logos should be 300–400px wide as a file, displayed at 150–200px via the HTML width attribute — this 2x approach keeps logos sharp on Retina screens. Banner images max at 600px wide. Keep all signature images under 100KB combined. Oversized signature images slow delivery and can affect spam scoring.
| Image type | File dimensions | HTML display width | Max file size |
|---|---|---|---|
| Company logo | 300–400 px wide | 150–200 px | 30KB |
| Headshot photo | 200 × 200 px | 80–100 px | 25KB |
| Banner / promotional | 1200 × 300 px | 600 px | 50KB |
| Social media icons | 40 × 40 px each | 20 px | 3KB each |
The Retina fix most signature guides skip
If your email signature logo looks blurry on iPhones, MacBooks, and any modern high-DPI screen, the cause is almost always the same thing: you uploaded a 200px logo and displayed it at 200px. On a Retina screen, 200 CSS pixels require 400 actual pixels. Your 200px file gets stretched to fill that space — blurry.
The fix is to upload a file at 2× the intended display size and constrain it with the width HTML attribute:
<img src="logo.png" width="200" alt="Company logo">
Here the file is 400px wide but the width attribute forces it to display at 200px. On a standard screen, it's perfectly sized. On a Retina screen, the extra 200 pixels fill in the detail — it looks sharp. The tradeoff is a slightly larger file size, but at 400px a PNG logo is still well under 50KB.
Why large signature images hurt deliverability
Spam filters use a text-to-image ratio as one of many signals. An email where the signature contains a 500KB banner graphic and the body is a two-line message has a poor ratio — the message is mostly image, which is a pattern shared by bulk promotional emails and phishing attempts.
More directly: every image in your signature adds to the total email size. A 200KB signature image means every email you send starts at 200KB before you've typed a word. For high-volume senders, this increases bandwidth costs and slows delivery queues.
Corporate mail systems sometimes have inbound message size limits. A 10MB limit sounds generous until three people reply-all to an email thread and each reply carries the full signature image again. A 100KB signature image stays well under any corporate policy.
Keep all images in your signature under 100KB combined. For a logo or banner, compress to under 100KB before hosting. For a headshot photo specifically, 80KB is the practical target — it looks crisp in the 150–200px display width most email clients render at, and adds almost nothing to the email's total size.
Host images, don't embed them
There are two ways to include an image in an email signature: host it on a server and reference the URL, or embed it directly in the email as a base64-encoded inline attachment. Almost every guide recommends hosting. Here's why embedding creates problems:
- Email size: A 50KB PNG becomes roughly 67KB as base64. Multiply by every email you send.
- Spam filters: Embedded images bypass email clients' image-blocking features in ways that some filters flag as unusual.
- Outlook: Some versions of Outlook on Windows handle embedded signatures unpredictably — displaying them as attachments rather than inline.
Host your signature images on your company website or a CDN. The image URL can be https://yoursite.com/assets/email-sig-logo.png. The recipient's email client fetches the image from your server when they open the email. This is how every professional email signature tool works.
Format choice for signature images
PNG is correct for logos, icons, and anything with transparency or text. PNG handles sharp edges without the JPEG fringing artifacts that make small text look fuzzy. A PNG logo at 400px wide is typically 10–40KB depending on complexity.
JPEG is correct for photographic headshots. A headshot at 200×200px as JPEG at 80% quality is around 8–15KB — efficiently compressed for photographic content.
Avoid WebP for email. Major email clients — particularly Outlook on Windows — don't support WebP. Your logo will show as a broken image for a significant portion of business recipients. PNG is the safe choice.
What to do if your signature image isn't loading for recipients
The most common causes:
- The hosted URL is wrong — test the URL in a browser tab before setting it in the signature
- The recipient's email client blocks external images by default — this is normal behavior in Outlook and Apple Mail; images only load when the recipient clicks "Load images" or adds you to their trusted senders list
- Corporate firewall — some companies block all external image loads in email
You can't force images to load in recipients' email clients. Design the signature to work without images by including your name, title, and contact information as plain text alongside the image — not as text burned into the image itself. Keep images under 100KB so when they do load, they load fast.