To answer more on to the question down load a tile editor (google Tile editor download)
get yourself Crystaltile2 or tiled or tile molester
For CT2 open your rom
view the file system find file with image that needs replacing
fiddle with settings til image appears insert image and save rom
change width to 256 and height to 192 change the bpp til the image appears may need to load pallete for image go to pallete beside the properties pane import pallete from image (your new image)
you may have to change the offset of the image to view it correctly so play with that too of the image still doesn't appear quite right
then import you image from edit import image then place image where necessary