racketboy.com

That splotch is strange but probably has always been there.

I've heard of people having graphic garble issues if even one of the pins on the 62-pin connector (cartridge slot) is corroded/gunked/damaged. Replacing the 62-pin connector sometimes fixes this. Of course cleaning it thoroughly first is recommended.

Unfortunately it isn't impossible that a PPU or video RAM chip is internally damaged.

That's probably the first thing I'd do, make sure that your pin connector is solid. Clean it up good, and make sure it doesn't appear that the pins aren't bent in a lot. While it's more common on NES systems, one has to think the SNES isn't immune as well.