You can use images on both posters and billboards; but the license you need might be different.
Posters: You can use images for advertising and promotional posters and point of sale pieces. The Standard License plans (Subscriptions and On Demand) covers up to 500,000 copies, while the Enhanced License covers unlimited copies.
Billboards: Billboards differ from posters in that a billboard is real estate or other property, like a vehicle, that is used to reach a larger number of viewers. Billboards (both print and digital) and similar signage are covered by the Enhanced License for unlimited views, also referred to as impressions. The Standard License limits the total number of views to 500,000.