Shortly after you have returned Kate her book you will have to exit the girls dorm. Once you have left the dorms, head right and make your way to the trash can that is next to a bench. Max will take a piece of bread out of the trash can and put it on the bench. After you have done that, walk way from the bench and wait for the squirrel to get on it and start eating the bread. Once the squirrel has gotten on the bench walk up to it and you will be prompted to take a photo. After you have taken the photo the achievement will unlock.