It’s been a while since I used it but I would stick with CS2 rather than consider replacing it with the GIMP. If you shoot in your camera’s RAW file format (and you should if you have a DSLR) then the 16-bit color features of CS2 will be sorely missed when switching to the GIMP. For photography purposes, I would consider getting Adobe Lightroom before CS5, the GIMP, or even a new lens.