Yes you can! Unlike logo projects, all other design projects may contain stock art, clip-art or royalty-free images.
There are some simple guidelines you must follow if you want to include stock or clip art in your non-logo submission:
1. Make sure that the license allows you to do so. Read it well. If you don't understand the terms of the license well enough to be absolutely certain that it’s safe to use, email customer support for the site you got the stock art from.
2. Select the right disclaimer when posting your entry so your client knows what they’re working with. Misrepresenting creative property that you do not own as something you do own is a big no-no.