Format Reference
WebP and AVIF have effectively no support across email clients. Despite their adoption on the web, email is still locked to JPG, PNG, and GIF. APNG has limited support in Apple Mail only.
Image Format Support Matrix
Supported: JPG ✓, PNG ✓, GIF ✓ (all major clients). Limited support: APNG ✓ (Apple Mail only). Not supported: WebP ✗, AVIF ✗, SVG ✗.
Why WebP Doesn't Work in Email
WebP is the modern image format on the web, but email clients haven't adopted it. Only Apple Mail supports WebP, making it unreliable. Stick to PNG and JPG for universal compatibility.
Image Blocking Reality
Most consumer email clients load images by default. Corporate firewalls and security gateways still block them, and some users opt out for privacy. Always include alt text and meaningful fallback text so the message stands when images don't load.
ALT Text & Accessibility
Provide descriptive alt text for meaningful images. Use `alt=""` for decorative images. Alt text is essential for accessibility (EAA requirement) and serves as fallback when images are blocked.
Image-to-Text Ratio & Spam Filters
The long-standing advice is to maintain a 60:40 text-to-image ratio because image-heavy emails get flagged as spam. That rule comes from rule-based filters and it no longer predicts consumer inbox placement. InboxEagle's analysis of 774,828 emails found image-heavy emails had the best placement of any content type — 81.92% inbox and 18.08% spam, against 22.28% for balanced templates.
Still include meaningful body text and a plain-text part, for rendering, accessibility and the gateway filters that do score this way. Just don't sacrifice a converting design to hit a ratio that placement data doesn't reward.
Retina (@2x) Image Sizing
Create images at 2x the display size (600x600px display size = 1200x1200px source). Set width and height attributes to the display size. Most email clients handle retina scaling automatically.
GIF Animation & Fallbacks
GIF animation is supported in most modern email clients. Outlook 2007-2019 doesn't support animation; use the first frame as a static fallback. Keep GIF file sizes under 100KB to avoid blocking.
Background Images & VML for Outlook
CSS background images work in some clients but not Outlook. Use VML (Vector Markup Language) with `<v:rect>` tags for Outlook support. This requires MSO conditional comments.