Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
iTunes will accept pretty much any image file as the album art (if this what you're trying to do). Just open the info for the music track(s) from iTunes and drag the image file from Finder onto the album art block. This assumes you have the image file you want to use of course. If you need that created see Butters' answer above. Gimp is an alternative for image creation though more hassle on the installing.
 
Thanks so thats what I wanted to know. I drag the image onto the audios info.

How can I select a bunch of audio files and make them under one album with one cover arT?
 
1. highlight all the files you want in one album

2. right-click > get info

3. in the bottom left of the window, set "compilation" to YES

4. wait til iTunes makes the change. You're done.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.