Grit for Virtual Boy is a slightly tweaked version of the versatile bitmap conversion and tilemapping tool by Jasper Vijn for GBA/NDS. It converts 4 colour indexed images (bmp, png, gif, etc.) into 2 bpp 8x8p tiles and BG map data for copying directly into VRAM.
For full instructions see the manual: http://www.coranac.com/man/grit/html/grit.htm.