Sites with tons of high quality official art: very useful for custom covers!

[SMMM]

http://www.kawaiidream.net/

Here is one I use very often.  Requires registration, but it's well worth it for the amount of art they have for hundreds of different games.  If you know of any others, even if it's for a specific game/series, post em!

[SMMM]

http://www.emugifs.emuita.it/titlelogo.htm

This site has many title logos from mostly arcade games.  They're already transparent, so you just need to pop em on.

[KaiserWAVE]

Now that's an awesome site. Thanks for pointing me in that direction!

[SMMM]

http://www.animepaper.net/gallery/

Even better.  This site has a HUUGE collection of very high quality (2000px) wallpapers/scans from magazines or artbooks.  Only thing is that it relies on its own currency called "paper".  You submit artwork/wallpaper, you earn paper.  Then you use the paper to spend on downloading art/wallpaper.  ..or you can donate $6 and get a ton of paper, plus other benefits.

[Doom]

As stated in the company logos topic, LOADS of official art, including logos!
http://vgaa.emulationzone.org/

[Vt102]

Found this site wile surfing the net.

If you like arcade stuff.. It's got some nice images in high quality

http://www.localarcade.com/arcade_art/