2 ways to do that, each with their own drawbacks. You can edit the GFX in LM by extracting and inserting the GFX, then opening up the 8x8 editor. select a tile, redraw it, right-click to paste it (Can't overwrite animations), and then press F9 and reinsert the GFX. You can't, with this editor, edit animated tiles or sprites. To do this, you must download
YY-CHR. In this editor, you can edit all GFX. Extract them and set YY to 4BPP SNES. The palettes will be the problem here. to get the right palette, play your level in ZSNES and make sure you save in your level. Next, push the button in YY that is at the top right and looks like a game cartridge. Load the save state in there and draw. Notice that you will have to scroll through the palettes to be able to find the right one. After redrawing and saving, insert the GFX in LM.