The discussion of reuse or appropriation of other peoples' images or work in the making of a new art work is well discussed in Randy Kennedy's January 1, 2012 NY Times article "Apropos Appropriation "
The internet makes appropriation all the easier. The flood of online images used as source material will never be dammed. Artists are all over it. It is 21st century fodder. Penelope Umbrico's work made from googled images is one example( I saw this one at the San Francisco Museum of Art). By using images that weren't created by people with a monetary interest in copyright such as corporations, designers, or other artists her work smartly sidesteps the issues raised in the article. A different take on use of net generated art is a review from the Economist blog of a recent show at Pace Gallery "Social Media".
When I look at the jewelry of artists that use images I often wonder when no credit has been given if they produced the images themselves, more likely they appropriated them. Jewelry is on the farthest most periphery of this discussion perhaps, but it is interesting to follow the topic just the same.
Wishing everyone a Happy New Year.