Why do compilation albums sometimes show up as 100s of identical album covers?
It's possible the "Part of a compilation" meta data wasn't set for these tracks. You can group them together as a single "album" in iTunes by:


Selecting all the tracks you want to come from the same album (use Cmd-Left Click to select multiple, non-contiguous tracks) 
Hitting ⌘-I to bring up the meta-data editor window and then: 


For the Album Arist field enter: Various Arists
For the Album field enter the name of the compilation album
And then make sure the Part of a compilation option box is checked

Click OK to apply the changes to all the tracks


You should now see them grouped together in the grid view as one album, all showing (hopefully) the same cover art. If they're not showing the same cover art:


Find the cover art you like.
Copy it to the clipboard with ⌘-C
select all the tracks again and open the meta-data editor window.
Click once in the Artwork window and hit ⌘-V to paste the cover art to all the tracks.
Click OK to add the cover art to every track in the compilation.