Locate where the compressed graphics are (I'm pretty sure they are; shouldn't be LZ-whatever). Decompress, edit, recompress. Repoint if data is larger than before. If nothing fancy is going on, this should be all that's necessary :/
Unless they're already decompressed, though I doubt it. Just get a generic graphics editor for ROMhacking--I use YY-CHR, others use TileMolester, and so on.