From the Super Mario Wiki, the Mario encyclopedia
Toadbert's Drawing is a key item in Mario & Luigi: Partners in Time. It was drawn by Toadbert, who lost his memory, and sketched the memories he still had left. It can be viewed inside Stuffwell. It is drawn very poorly, and has dirt and mud over it. The bros receive it inside Yoob's Belly in Yoshi's Island.
Later in Star Shrine, the player has to clean off the mud and dirt using the stylus, revealing two Princess Shroobs. Just as Toadbert is about to explain, Elder Shrooboid comes out of nowhere, and turns Toadbert and Kylie Koopa into Shroob Mushrooms. The two princesses are later revealed to be Princess Shroob, and Elder Princess Shroob. The item is never used again, although it remains in the brothers' inventory with the in-game description "Toadbert's Odd Drawing".