An email campaign is judged on a screen the sender never sees, on a connection the sender does not control, from a mailbox that has to download every image before the reader scrolls past it. That makes image weight in email a different problem from image weight on a website. On a website a slow image costs you a fraction of a second and the browser caches it. In an email the image is fetched fresh, by every recipient, every time the message is opened, and a fat first image means the top of the campaign renders as empty boxes while the reader decides whether to keep scrolling. Two constraints set the target. The template width: Mailchimp's classic templates are 600 px wide and its new builder is 660 px, and it recommends image widths of 600-1200 px for the classic builder and 660-1320 px for the new one, so anything wider than about 1320 px is pixels no reader will ever see. The mailbox: Gmail clips messages whose HTML exceeds 102 KB, and Outlook.com caps a whole message at 25 MB. Skwish compresses and resizes in the browser with a before and after per file, so you can bring a campaign's images to a consistent weight before they go into the builder.