Before you participate in a project, please check the “Guidelines, regulations, comments” section of the brief. If you are permitted to use Third party material, there you will find whether you must OWN all copyrights.
For more information, you can download the Third party material 101 attachment below.
If you design all elements of your entire submission, you automatically own all copyrights.
In case you are not the creator of the whole submission you need to own the third-party material, meaning, buying the license from the stock provider.
Read more here.