Let me show you how GameSharks work:
GameSharks basically mess with the coding on the games. You see, the cartridge plugs into the top of the GameShark, and the GameShark plugs into the N64. The GameShark gets all the info from the cartidge, and then "overwrites" some of the data to a fixed code that you put in through Hexadecimal format.
Now, none of the coding that the GameShark edits has to do with textures. Even if it did, it would take a LONG time--meaning several years--to find the right codes, and there would be a LOT of codes--meaning thousands of lines.
And, the biggest reason of all--how would you store the textures?
Hopefully now you know.