It’s the client’s responsibility to buy any third-party elements, including stock art, so that they can own the licenses to those elements, and avoid possible licensing issues.
If you use stock art in a design you’ve submitted to a contest, we need you to:
tell the client which images are stock art, using the licensed content declaration
use the watermarked versions of those images in your design.
Once the client chooses the winning design, they’ll purchase the stock art and provide it to the winning designer so they can complete the design.