conkan, you may find Fair Use (FAQ) | U.S. Copyright Office of interest. Specifically, the sections on Derivative Works, http://www.copyright.gov/circs/circ14.html , and Fair Use doctrine, http://www.copyright.gov/fls/fl102.html .
Section 107 also sets out four factors to be considered in determining whether or not a particular use is fair:
the purpose and character of the use, including whether such use is of commercial nature or is for nonprofit educational purposes;
the nature of the copyrighted work;
amount and substantiality of the portion used in relation to the copyrighted work as a whole; and
the effect of the use upon the potential market for or value of the copyrighted work.
The distinction between â€œfair useâ€ and infringement may be unclear and not easily defined. There is no specific number of words, lines, or notes that may safely be taken without permission. Acknowledging the source of the copyrighted material does not substitute for obtaining permission.
If you really want to dig, check out Copyright Law of the United States | U.S. Copyright Office !
Copyright permission is granted by the original holder of the copyright. If a photograph involves copyrighted subject material, you have to seek permission from the copyright holder of the source material as well as from the photographer for the use of their work.
If you are unable to receive explicit permission from a source, whether from a direct request or as part of a licensing agreement (when purchasing stock images, for example), it is probably not worth the potential risk and inconvenience of using something and getting caught. Aditionally, neither alteration of a work, nor the inability to determine the copyright holder, absolve you of responsibility to the copyright holder. This is especially important to consider for commercial usage, such as yours.
For minimum hassle, I suggeset you look into/pony up some cash for royalty-free images! There are plenty of sources to be found through Google. Hope this helps!