Images in emails are one of the most common causes of poor email performance — slow loading on mobile, triggering spam filters, and exceeding attachment size limits. Most email marketing guides tell you to "keep images small" without explaining exactly what small means or how to achieve it.
This guide covers the exact target sizes for email images, which format to use, and how to compress any image for email free in seconds.
Why Image Size Matters So Much in Email
Email clients handle images very differently from web browsers:
Mobile loading: Over 60% of emails are opened on mobile devices. Mobile connections are slower than WiFi, and large images cause emails to load slowly — or not at all if the recipient's mobile data is limited.
Spam filters: Many spam filters flag emails with high image-to-text ratios or very large image file sizes. Smaller, optimised images are less likely to trigger spam detection.
Attachment limits: Gmail limits email attachments to 25MB total. Outlook limits vary by server configuration but are often 10–20MB. For email newsletters sent via marketing platforms (Mailchimp, Klaviyo, Campaign Monitor), inline images load from a URL — but large images slow email load time and hurt deliverability scores.
Double compression: Email clients and marketing platforms often recompress images when displaying them. Starting with an already-compressed image gives you more control over the final quality — rather than letting the email client compress aggressively.
Target Image Sizes for Email
Which Format to Use for Email Images
JPG for photographs: Use JPG for product photos, hero images, and any photographic content in your email. JPG's lossy compression achieves very small file sizes for photographic content — typically 50–80KB at 600px wide with 60% compression.
PNG for logos and graphics: Use PNG for logos, icons, and graphics that require sharp edges or transparency. Note that many email clients do not support PNG transparency — the transparent areas will display as white in most email clients. If your logo has a transparent background, place it on a white or coloured background before converting to PNG for email use.
Avoid WebP: WebP has excellent compression but is not reliably supported in all email clients — particularly older Outlook versions. Use JPG and PNG for maximum email compatibility.
How to Compress Images for Email Free — Step by Step
Step 1: Resize first, then compress
Resize your image to the dimensions you will actually use in the email — typically 600px wide for a full-width email image. There is no benefit to including a 4000px image in an email if it displays at 600px. Resizing alone often reduces file size by 80–90% before any compression.
Step 2: Open the free image compressor
Go to webassetsuite.com/compressor. No account, no limits, supports JPG and PNG.
Step 3: Upload and set compression to 60–70%
For email images, 60–70% compression is the target range. This achieves file sizes well under 100KB for most 600px-wide images while keeping visual quality acceptable for the small sizes email images display at.
Step 4: Check the output size
The compressor shows the exact output file size. For email, target:
- Header images: under 100KB
- Product images: under 80KB
- Logos: under 20KB
If still above target, increase compression to 75–80%.
Step 5: Download and use in your email
Download your compressed image and upload to your email marketing platform or insert as an attachment.
Compressing Images for Email Attachments
For images sent as email attachments (not embedded in newsletters), the rules are slightly different. Most email attachment limits are 10–25MB total, but large attachments are slow to download and annoying for recipients.
Recommended approach for photo attachments:
- Compress to 60–70% using the free image compressor
- Target under 500KB per photo for casual sharing
- Target under 1MB per photo for professional/client work
For multiple photo attachments: Use the batch image compressor to compress all photos at once and download as a ZIP file — then attach the ZIP rather than individual files.
Email Image Best Practices
Always use alt text: Many email clients block images by default until the user clicks "Display Images." Alt text ensures your message is readable even when images do not load.
Host images externally, not inline: For email newsletters, images should be hosted on a server (your email platform handles this automatically) and loaded via URL — not embedded as base64 data within the email HTML. Embedded images massively increase email file size.
Test on mobile: Always preview your email on mobile before sending. An image that looks fine on desktop may be too large or poorly scaled on a 375px-wide phone screen.
Consistent dimensions: Use the same dimensions for the same type of image throughout your email template — consistency makes emails look more professional and loads faster because the browser knows how much space to allocate before images load.
Compressing Images for Specific Email Platforms
Mailchimp: Recommends images at 600px wide maximum, under 1MB. Use 60% compression to get product images under 100KB at 600px width.
Klaviyo: Same guidelines as Mailchimp. Klaviyo's CDN serves images at appropriate sizes but cannot reduce file size — compress before uploading.
Gmail signatures: Keep signature logos under 20KB. Use PNG for logos on white backgrounds, 200px wide maximum.
Outlook signatures: Outlook handles images differently — use JPG for signature images rather than PNG, as some Outlook versions render PNG transparency incorrectly.
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