I came across a discussion which included a thought provoking link on the perils of saving company money by using low-cost images:
The article shows how the same image can be used over and over, in a variety of contexts, companies and countries. As long as each ad designer is unaware of the other examples, fine.
The discussion thread, containing lots of interesting comment from professional photographers, was here.
What is microstock? See here. How can one track where an image is used, e.g. to find copyright violations? Try Tinyeye.