Just to follow up on the initial question a bit more, the chocobo graphics in question are compressed graphics, and thus needs to firstly be decompressed with good ol' PeerSpriteViewer (PSV), which allows you to decompress & export compressed graphics as a .bin file once you input the correct offset info (using the ff6 ROM map), in order to then edit it with YY-CHR. Then you can recompress the graphic back in, with PSV, making sure that it is the same size or smaller (it shows the size), or else bad things happen!
We are born, live, die and then do the same thing over again.