Hello jeffmdeen
I don't have any iNtel Mac, so I verify try this, but, if you don't get better help here, try using an image editor app like PhotoShop or PhotoShop Elements to open or create the image you want to use.
Make any desired changes and cut out the openings wherever you want.
Then use the File > Save As menu to save the file as a "transparent" GIF or PNG file. See Help in your image editor software If you need details instructions for saving the "transparent" file.
EZ

Jim
PowerBook 1.67 GHz w/Mac OS X (10.4.11) G5 DP 1.8 w/Mac OS X (10.5.1) External iSight