The header is the one image every recipient sees, and it is almost always oversized for the slot it lands in. Mailchimp states its legacy templates are 600 px wide and its new builder templates are 660 px, recommending images of 600-1200 px and 660-1320 px respectively — the upper half of each range is the Retina case, where the image is stored at double the display width so it stays sharp on a high-density phone screen. That gives three sensible targets and one wrong one. Right: 600 px for a legacy template, 660 px for a new-builder template, and 1200 or 1320 px when the header must look crisp on Retina. Wrong: the 3000 px banner exported from a design tool, which the template scales down to 600 px while every recipient still downloads all 3000. Mailchimp's own guidance points the same way, recommending against uploading images significantly larger than the content area. Resize to the slot, then compress — and if the template supports Retina, resize to double the slot and compress to a byte target that keeps the file small anyway. Skwish does exact-dimension resize and compression in one browser pass.