It’s the contest holder’s (customer) responsibility to buy any third-party elements, including any stock art. This will ensure the contest holder takes ownership of the stock art that's being used within the design they choose.
If a designer uses stock art in a design, the designer will:
tell the contest holder which images are stock art, using the licensed content declaration
use the watermarked versions of those images in their design entries
Once the contest holder chooses the winning design, they will need to purchase the stock art and provide it to the winning designer so they can complete the design.
If a contest holder does not want any stock art used within their design, they must declare it in their design brief. This ensures that the designers have the right expectations when starting their designs, from the beginning.
If you are okay with using stock art to enhance the designs being shown to you, it's best to set a budget for the designer(s) to use.