I make things harder than I have to. I first search for large, clean versions of the images with Google Images, then, if I can't find anything of appropriate quality, I'll scan the cover myself. I scan it in large, then scale down to 600x600 (or if it's not a perfect square image, 600 would be the max for either dimension) with almost no compression. I'm weird and think that even 600x600 might not be enough, just because it would still look scaled if I made it fullscreen on my 1080p display. But since it's usually just a portion of my MediaMonkey window, I settle. A large part of my collection either has no album artwork or lesser-quality scans from other people. It's of course a goal to get everything up to snuff over time, but I'm not being adamant about reaching it. One of these days....
