Well, garsh, thanks for linking to my site!
Anyway, you might find it helpful to get GIFconverter, and know that your image needs to be 512 x 342, not 512 x 384 as I said in the readme. Be warned that the normal way of making startupscreens in ResEdit won't work. You must use GIFconverter or SuperPaint, I prefer the former.