Hey! I've seen your name around. I'm assuming you're ok with the sprite not scrolling. To draw an image to the background, you'll essentially need to do 2 things:
1. Load the image Tiles into VRAM.
2. Copy those Tile's IDs into the Background Map. (Use FuncCoord to load background map address into hl)
You also might want to erase some of the old titlescreen behavior to get rid of the scrolling pokemon and stuff, which is what I did.
Here's a link to the changes I made when I updated the titlescreen:
https://github.com/huderlem/maize/commit/2e86af3bae36ea6b2ee5a60cbfd0ab7d3d642f5c
I apologize for any lack of commenting and badly-named labels. (I'm looking at you ".loopdeeloopMaizeGfx".) This is basically what I did in that change:
1. Cleared original titlescreen functionality.
2. Load big ampharos graphics (TitleScreenPic) into VRAM.
3. Load graphics for ampahros's closed eyes (TitleScreenBlink1Pic and TitleScreenBlink2Pic) into VRAM.
4. Load ampharos picture tiles into the background map (using FuncCoord to get tilemap address into registers hl).
5. Load Maize version text into the background map.
6. Randomly make ampharos blink and load the closed-eye tiles into the background map.
Let me know if you have any more questions!