Bring Out the …

GIMP (aka GNU Image Manipulation Program) is an opensource image editing program that can come in handy when you need the functions of Photoshop but you don't have Photoshop. It runs on Windows, Mac, and Linux.

The interface may not be as polished as other programs out there and startup can be a little slow, but there is a bounty of documentation and information available here and here. Did I mention that it's free to download, use, and distribute?