JellE, here is how to add the artwork to MAME OS X:
-Go to your home folder (whatever your login name is)
-From there, go to "Library" -> "Application Support" -> "MAME OS X"
-Drag the invaddlx.zip artwork file into the "Cabinet Art" folder
I have no idea how to crop or disable the bezel art, however. If someone does, please let me know.
Also, in the MAME OS X directory above, the "Screenshots" folder is where in game screenshots get dumped when you use the F12 key to take one.