I don't know if any good tools for this task exist. But if I had to do this, I'd write a plain Ruby/Gosu tool that lets me drag rectangles on the graphic and cuts up the image, rearranging it into a grid of sprites where each image has the same measurements. Then you can either load it with Image.load_tiles (easy), or as a several 1024x1024 images with Image.new and use .subimage() to load specific sprites (that'll be a little faster).