all!

I'm a total newbie here so I please forgive me if I am posting in the wrong area or posting a question that was already answered.

I have been given the responsibility of doing a very simple Photoshop edit for a long distance friend's website: putting an image inside of text. They have asked me to use one of their photos to put inside the text. 

I'm new to graphic design (not schooled) so I just want to cover my bases. The photo they gave me is really well done, like a professional photographer, except I didn't realize they even did photography. How do I know that the photo they are giving me is their photo that they own the rights to? I don't want to use a photo that's not theirs and get in trouble later on. 

I want to provide this service to more people (putting image inside text) but should I just stick to my own images/stock/free for reuse images?

Thanks in advance!