I had the same problem. The file I originally had was named iTunesArtwork.png (Apple's docs said to use a PNG - I've since figured out that it works with either a PNG or a JPEG, but in my case the PNG is considerably smaller). Using the Finder, I renamed the file to just "iTunesArtwork", but it didn't work when put into the built application. In addition, somehow XCode remained convinced that the "real" name was iTunesArtwork.png when I tried to import it, even though it showed as just "iTunesArtwork" through the Finder and through the import process.
What I finally did was open up a terminal window and go look at the file. It turns out that even though Finder wasn't showing the extension any more, the underlying file still had the extension - Finder was just hiding it. One clue was that the file's icon was still showing up as a graphic.
When I used the terminal window to rename the file
Code:
mv iTunesArtwork.png iTunesArtwork
then, in the Finder, the icon for the file changed from my graphic to a gray rectangle. THAT file worked just fine when dropped into my built application. I was even able to then import it into XCode so that it's automatically included in the build.
Moral: Don't trust the Finder when it comes to removing extensions on files. Go down to the bare metal and do it for sure.
