As I’m sure you know, the US has strong copyright laws. That means, among other things, that if you take a picture, that picture belongs to you, and other people cannot use it without your permission. If they do, you can take them to court and win damages.
Now, to be sure, there is a “fair use” exemption in the copyright laws. Do not take this as legal advice, but “fair use” basically means that if you don’t benefit too much nor violate copyright too much, it’s ok.
For example, if you take a picture of Obama photographed by an Associate Press photographer for the purpose of taking Obama’s head and putting it on a picture of your aunt and uncle Cindy and Louie so that it looks like Aunt Cindy is actually married to Obama and then distributing that photo among your cousins just for the LOLZ, that’s ok.
However, if you took that picture of Aunt Cindy and Obama and put it on T-Shirts to sell, that wouldn’t be ok. That likely wouldn’t pass muster under the “fair use” doctrine.
Fortunately, thanks largely to the efforts of constitutional scholar Lawrence Lessig and others, we have something called the “Creative Commons.” This allows people who produce creative material to declare that their material may be used, even for commercial purposes, as long as they are credited as the authors of said material.
Here’s a great search engine for material that is covered under the Creative Commons License. http://search.creativecommons.org/ Enter your search terms, select the search engine you want to use, and off you go!
Be advised, however, that if you want to use the material for actual commercial purposes, that you should contact the author or creator and verify that you can do so or negotiate the terms under which you can do so.