Let me give some input here. DPI means nothing unless you're working with a physical print. I've printed many 36 inch images at 140 to 160 dpi and they look very good. For a book cover though, I would not go under 300 dpi.  Also, you can interpolate an 800 x 600 pixel image to 8000 x 6000 if you wish, but all you're doing is magnifying fine detail, which is fine if you want a giant blur. It's all up to you the artist. There's really no rules.  Personally, I'd like to print a 96 x 60 inch picture at 400 dpi but my hardware can't handle that!

Also, if your image has smooth, fine gradients stick with a lossless format like TIF, otherwise a low-compressed JPEG looks just fine.

"An Example is worth Ten Thousand Words"