Define the behavior that should have a gimp plugin that imports/exports from/to the new format tuxpaint uses(png with embedded data in custom chunks) and xcf. See do_png_embed_data function in tuxpaint.c for details and documentation. Ask in the tuxpaint devel mail list if you have doubts about it. Write a document with all use cases, the suggested behavior and reasone it. If you see various valid alternatives, present them and reasone what do you think is the best.