This program can decompress and dump most of the graphics in Yoshi's Island, so that they can be modified by your SNES tile editor of choice. The program can then be used to recompress and insert the graphics back into the ROM.
Instructions on using the program are within the readme.txt file included in the zip file.
Ycompress - Yoshi's Island Compression Tool (WinZip File)
Size: 46K zipped, 91K unzipped
Minimum OS: Win95
Released: September 27, 2004.