From the Super Mario Wiki, the Mario encyclopedia
Peach's Photo is an item found exclusively in Mario & Luigi: Dream Team, and the fourth item collected during a series of item trades pertaining to a sidequest in Wakeport. As the name suggests, it is a picture of Princess Peach. Mario and Luigi are given this item from a Yoshi-loving Toad just over to the west of the entrance to Pi'illoper's house after giving him a Yoshi Egg.
Mario and Luigi must then head to the Mole Mario panel puzzle just up north to give this item to a Brock who claims to be a big fan of Peach, who will thank them and give them the Book of Ruins in exchange.