I'm an expert in this.
I use an app called BIMBO v1.3 to convert my 240x160 256 color (indexed) bitmaps into .bin files. Leave the Graphics mode at 3, and choose your Dithering preference. Export it to a .bin file.
Then when you are in the Goomba Front GUI, go to select a Splash Screen file and where it says *.raw, change that to All Files (*.*) and choose the .bin you made.
And that's it! Compile your ROM, and it should show for about 2 seconds at boot.
One tip I do though is I set the brightness up by 1 in BIMBO, and that way it looks correct on the GBA screen.