MailJet - image sizes
Our recommendation for image sizes to use for the banner headings in MailJet is approx 1200 pixels x 320 pixels
MailJet will display these images at 100% full bleed ( when the printed color & images extend all the way to the edge of the paper)
Some clients have supplied images that are : 900 x 400 pixels, and they still show really well, at full bleed, edge to edge. And that gives a slightly deeper picture, eg: the Bills Tours ones.
Our recommendation is that your images a) branded, b) optimised, c) narrow horizontal/landscape