Looks like it's a couple of good sites for the covers, but I have never used them.
Most of my collection is an actual scans from physical copies taken by me or sent to me by friends.
So the standard resolution for me is 300 dpi (in fact I scan everything in 600 dpi, but not all covers deserve saving in such resolution).
Also as part of my collection can be considered what collecting dust on my shelves. In the past years I regularly bought DVDs (mostly anime) and gathered big enough collection to get the problems with finding a place for its storage
Lately, I rarely buy DVDs (only once in 2-3 months) and if possible prefer Blu-ray. But about two years ago, I began to actively collect CDs (also mostly anime and game music, and usually old and rare). Sometimes I buy console games, also mostly old and rare.
As an example I can give you a couple of my scans
CD and
DVD size (300 dpi).
And if you're interested in higher resolution, I have both in 600 dpi
