Yes! But, ANY and ALL third party images must be declared according to our Third-Party Material Policy.
In Contests where third party material is permitted, use the use the declaration tool to properly declare them.
In 1-to-1 Projects and in Contests where third party material is not allowed—such as logo, illustration, and icon/button contests—there is no option in the interface to declare these materials. Therefore, you must inform the client somewhere in the description of your design that the images you used are for presentation purposes only and are not included as part of the finalized design.
Be aware that in a Contest, this method of declaration is private between you and the client, which may lead to other designers reporting your design. However, if your design is reported, you can challenge the report by proving that you informed the client about this and clarified that the image is not part of the contest deliverables.