I am very fond of Acorn, the inexpensive but feature rich graphics program for Mac by Flying Meat. I have spent many happy hours in Acorn fiddling with images for my apps, including the trump cards for Grimes and Racing Demon and the achievement badges in Grimes.
Another task I do in Acorn is putting together app icons. I have an unfortunate habit of putting elements of the icon too close to the corners. When the app is on a device the corners are rounded and the icon looks odd. To help me avoid this I have created a layer in Acorn that provides a set of guides to help me lay out the icon, and in particular to keep aware of the corners. I add this layer to my Acorn icon files and hide it when I don't need to see the guides.
There are similar things around the net in Photoshop formats. My Acorn equivalent at 1024x1024 pixels is here, feel free to use it if it helps you.
Happy Acorn iconing.