It’s the contest holder’s 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:
1. Watermark any stock art as such in the design submitted to the contest.
2. Inform the contest holder in the comments section about use of any stock art, using the licensed content declaration.
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.
Pro Tip: 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.