Originally Posted by tompe
Did you disable html2mobi's rescaling of the images in this procedure?
No, and so the last 5 images in the original MOBI file are all 480 pixels wide. Fortunately, this error was self correcting (because html2mobi and the Kindle both maintain image aspect ratios) and the 520x640 limit seems to be right. I attach the last 2 images (520x640 and 540x640) from a MOBI created using mobigen - which does maintain the original image sizes. These end up the same size on the Kindle as the original versions. They should in principle have higher quality (one less rescaling step), although this isn't all that apparent.