Yes, you can! However, in the case that you are selected as one of the top 50 photographers in one of the Photo Contests you will need to remove the selected photo(s) from these platforms.
This is because if you are successful in becoming a Hero, Agora or the brand sponsoring the Contest will have the rights to your image (purchased in the form of the $1,000 prize). For this reason, your image cannot be sold to anyone else during the selection process until the winner is confirmed.
If you are unsuccessful in being chosen as the Hero you may put your images back up for sale elsewhere!