COVER SITES:
Replace the Case
Formally known as Dagon's Den, which is a site that opened after the downfall of DVD Cover Art, you won't find a better place for high quality customs online. This site is on a credit or subscription service.
R1DB
Very similar to the above in quality, plus a great range of resources and converts from scanned material. This site is on a credit based service.
Generetic
Huge community and you can find almost anything, however lower in quality than the above sites. This is a pay per service or you can be granted access if you contribute.
Euphoric FX
A free site (for now), they offer a little bit of everything. Not a huge community, but if you're starting out, they are very helpful. Paint Shop Pro users, this is your best site for cover tutorials.
Rix Grafix
Ric Easton is one of the best cover creators out there and his site showcases his amazing work with flare. He offers his downloads free.