. I looked up Seashore and it's apparently written for Mac only, but is based on the GIMP codebase and 'is still undergoing development and may contain bugs'. If you're wedded to Photoshop, there is a mod of Gimp called 'Gimpshop' that rearranges the UI so it looks more similar to Photoshop. If you are starting from zero with any of these programs, the learning curve will be the same so I'd stay with Gimp. .