Easy to use, each billboard is numbered in the layers palette. Just enable the one you like, open the folder and you'll see red-marked layer (smart object) called "Double click Paste image" - so double click on it's thumbnail and it will open your billboard's artwork. Paste your image over mine (stretch to fit nicely) and save. It will update the billboard. There are optional shadow layers that you can disable if not needed. Each billboard has the optional Grunge layer that you can disable, or invert (ctrl+I) if your artwork is dark so it's white instead of black.