Omg. I still can't believe it, when I see it. When I have the time (school started back up) I am going to attempt to do this! Do you use the graphics editor that is built in SGDK2?
You can use the graphic editor. I gave it a spin and actually found it to be pretty darn useful. If there was anything that I would complain about, it is that it lacks a "preview" mode for tiled editing. It would be awesome if you could "preview" selected tiles to the top, left, bottom, and right of the tile that you are actually working on. These "previews" would not be editable, but simply graphical instances of graphic tiles you had already created. This would make it considerably easier to develop effectively tiling graphics. (since you would be able to line them up seamlessly with other tiles)
Aside from that, the graphics editor is very capable, and very good for sprite development specifically. It was wise to base SGDK2's graphics on the PNG format. It is the perfect graphic format for these types of 2D games. I tend to switch anti-aliasing off when I sprite draw, as having per-pixel control is very important to me. Also, I tend to zoom in to the maximum setting for individual sprite painting.