If you’re particular about resolutions, send at least a 1500×1500 pixels at 300 dpi photo for a 5×5 book or 2400×2400 pixels at 300 dpi photo for an 8×8 book.
When using a rectangular image as cover, make sure that when cropped to a square format, the important elements aren’t cropped out.
Allot at least a 1/2 inch bleed space around your cover image. And best to keep important elements in the center of the image.
See below a sample cover image. Cover images should have ample space beyond the red square, but the important elements should be within the white square.
