Modern mobile devices have retina (or high-density, or high-resolution) displays. A retina screen displays up to 3x the amount of pixels per point, which means the images would have to be up to 3x as large as the display size.
To ensure that almost anyone viewing your email on a mobile device sees crisp images, we would then need to save any image that displays on full width on mobile at 1242 pixels wide. Note that that would be the image width of every image that would stack to full width on a mobile screen.
Any images displayed smaller than full width on mobile would just need to be saved to 3x their display size. If, for example, you have an icon image that is set to display at 50x50 pixels in size, that image would just need to be saved at 150x150px for best display on mobile. (You could save all images at 1242 pixels in size, but this would mean unnecessarily large images that would take longer to download and use up more of their data plan).
This is our middle-ground, sweet spot we recommend for the average Drag-and-Drop user creating emails in Delivra. at 800px per image, the image is likely to render decently enough on retina screens without requiring a ridiculous file size.