I use Foobar2K and a skin that displays album art at almost 400x400, so it's sometimes tough to find sites which host good, quality album art.
I used cdcovers.to for several years but the quality there is awful.
About 2 years ago I switched to albumart.org. While they have a good selection, the chances of finding your album available at more than 300x300 is maybe 50/50.
As of maybe a year ago, I use a combination of two sites -
Between those two I can find just about any artwork I'm looking for and in exceptional quality. Does anyone know any other sites which host high quality album art only?