From the Super Mario Wiki
The Old Lithograph is an item that appears in Mario & Luigi: Dream Team. When the Bros. talk to Broque Madame while trying to find the Driftwood Jellyfish Sheets, she gives this item to them, which she found on a remote beach of Driftwood Shore. According to the inscription written in the Pi'illo language, pirates once roamed Driftwood Shore, stowing their loot in a blocked-off cave near what would later be the location for the Mole Hunt minigame, among them the Driftwood Jellyfish Sheets. The inscription also has a code, which Dreambert uses to open the entrance to said cave.