Well, first you have to figure out what it actually is, they you can rename it with the right extension. If it came from a Mac it it is probaly a JPG or a GIF or a EPS or a TIF, if it isn't a PCT. There used to be viewers like Compushow that would make a stab at tring to identifing a file from the first bunch of bytes, but I don't think anything does that anymore.