I always thought it was a picture of King Koopa from Super Mario 64. Remember when the Nintendo 64 came out, how Nintendo had that big ad campaign trying to convince us that koopas were dinosaurs, to make continuity with the crazy Mario Bros. movie? I thought part of that ad campaign made it into the Zelda game.

As to the picture in Majora's Mask, the first time I even walked in the vague direction of the marshes Tati pointed out the tree carving and a very sad, very long flashback happened. Did that not happen in your game? If so, that's a shame, it was really touching!