Cropping and resizing are the easy part for me. The larger the image the better. (I can always make something smaller, but if I try to make something bigger, the image blurs slightly.)

My method for these sorts of thing is to create a folder for the project and then start searching for all the images I can find and save them to that folder. I'll always end up with more images than I use, but in the editing stage sometimes the picture that I thought would be perfect just doesn't work for some reason and an alternate choice works instead.

Here's my gallery of cover art so you can see the sorts of things I've come up with before.