Before utilising any third-party material, please check the “Guidelines, regulations, comments” section of the brief. There you will find information about whether you are permitted to use third-party material in your submission in that particular project.
Because clients might want to license your work, it is important that your work does not utilise any material that is copyrighted by a third-party. Thus, you may use third-party material to present your idea but must clearly cite your source(s). Find out how to cite the material here.