Wouldn't compression reduce the picture quality?
Um, I don’t completely get your question, but SH has a set cover size of 250x350px.
I work with a canvas 2-10x that size in my drawing program, then save a resized copy in the required 250x350px size. There is no editor to crop the picture in the main site. If your image doesn’t have the same ratio, then it will be distorted into the ratio. If your image is too large in pixel then the site will compress the image. Therefore it’s recommended that you resize your cover image prior. The quality only goes down if you use jpg as file format, therefore I recommend png. But honestly, the difference is not dramatic.
Save the original size artwork for your glossary or put it in the first chapter.
You want a ratio of 2:3 tall for covers. Also, the characters' outlines are hard to see because they are blended in with the background. Ideally, there should be a title as well.
The ratio for SH covers is 5:7