http://wowprogramming.com/utils/artbrowser
translates into
<GameDir>\Interface\
But those files are packaged into the MPQ-archives so they won't be readily available right off the bat. You can however download them from Blizzard at the url below, just grab the artpack and the folder structure will look familiar from what you saw in the artbrowser. The files are .blp though, so you might need to get some software to view them (I believe XnView does a good job of this).
http://us.blizzard.com/support/artic...rticleId=21466