This is where the gray area starts.
While you may download free images for commercial needs like for an advertisement, you're going to lean yourself farther out of the window by using a free image to incorporate it into a composition that you are going to sell as your own.
Considering that the entire image won't be "yours", you might get away with it as it will most likely stay undiscovered by the original copyright holder unless you get the owner's consent to do so. I wouldn't give consent. Even on the few images I uploaded for free a long time ago at sxc.hu
Furthermore Shutterstock contributor terms prohibit this behavior where it states:
...
11.
Representations and Warranties:
You represent and warrant that:
...
c.
the Submitted Content and all parts thereof are owned and controlled by you, unencumbered and original works and are capable of copyright protection in all countries where copyright or similar protection is available;
...
If you ever get caught with that at SS, you're out.
I'm sure other agencies won't like to see this either.