First, be very careful on what media you allow on your site and how. For instance, Google Adsense will not allow any of its ads to be altered. So, if you only have space for a 250×250 ad. Then, you must find one to use that is that dimension or smaller. DO NOT use code from a larger ad and adjust to fit, or you will receive an email from the the Google team informing you that you have 72 hours to correct the code or you violate their terms.
Second, If you use pictures that you take and involve a person, then you must get express permission from the copyright holder to use, or be liable. This includes license Plates. Further, just because you alter the image, use captions or display an altered size, will still not release you from liability. You need Express Permission which is more or less a release form.
There are however thousands of free public domain images available on the net to pick from. Per Wikipedia:
“In the U.S., any work published before January 1, 1923 anywhere in the world is in the public domain. Other countries are not bound to that 1923 date, though.”
Thus, you can use any picture before January 1, 1923 from anywhere in the world.
“Public Domain Images”
Use quotes to tell Google to search for exact phrase.