It's likely a tileset with a separate tilemap of some kind somewhere. This is very common for tile-based graphics: the images are broken up into 8x8 tiles, and all those tiles are put into a tileset to draw images with.
For example, in Tomato Adventure (GBA), the "Game Over" screen has different text depending on if you have a save or not, and has a little animation of the main character swimming in ketchup. All the tiles for both versions and the animation are together in the same tileset.
In that case, and likely in your case, the game is just decompressing the lz77 data into VRAM tileset data at a certain address. Grouping multiple tiles together compresses better, and means less code for loading the tiles.
If all the tiles for the image you want to change are there, and none are reused by any other images, then you can just change them (leaving other tiles as they were) and it should work fine.