I like to start the secondary LXDE desktop in Easy Debian and then start Gimp (a photoshop clone) in there. I use the Maemo photo app to find the name of the photo I want to edit and then I switch back to Gimp to open the photo. I then do a white balance or whatever and save the photo with a new name. I can then go back to the photo app and flick back and forth between the before and after pics. When I like the results, I tag it and upload the picture to Flickr with sharing. (Example before and after pics)

I've noticed when doing some intensive tasks (editing a mask on a 10 megapixel picture from my DSLR or whatever) the N900 really becomes useless for other tasks... But I don't know any other handheld device that can run a photoshop-class application.