That's pretty complicated. You have to create multiple layers of callbacks that erase the screen, free graphics memory and then load your image. Then you have to create stuff the reads your image into VRAM. It takes quite a bit of code. I would recommend trying to disassemble the code that does egg hatching and see if you can replicate it. I usually use C for that sort of task. However, I have seen someone (on WAH) fade the screen out on all the background layers except the one that renders text, then display a message box. Then using some RAM offsets they were able to move the message box up and have a similar effect. You coud investigate that if you want a similar effect. You could also try changing the palettes to black manually and then doing that. I don't actually have IDA installed right now, so I can't give you offsets, but I hope I gave you enough to work with.