For uncompressed screens I use a dump of PPU memory and yy-chr for this kind of thing. Open the game on fceux, get to the screen, look in the nametable viewer in the debugger to make sure it doesn't use mid frame changes (so that this method works), then open the hex editor, dump the ppu memory to a file. Then open the file using PPU-SPLIT
, it will generate a CHR, a NAM and a PAL. Rename the PAL to DAT, then open the rom on yy-chr, open the DAT. Then open the nametable editor, load the nam, and done! Sounds more complicated than it is, but now you have it so you can edit tiles and see what they do, or right click the nam editor to find a tile in the chr table.