need help with city walls icon

[WesW]

I've put together a mod that divides the city walls improvement and the fort tile improvement (they use the same tile improvement). Problem is, I don't know how to design a new tile. All you would need to do would be to change the color of the original, or make it one ring of stones instead of two.

I have PSP and Microsoft Photoeditor, but neither one will open sprites. If anyone can tell me about a program I can get a trial version of that opens sprites, I would appreciate it.