Is there a way I can find out who owns the rights to an image I want to use on my web app? Are there any websites that provide that kind of information?
I will often try to find the source of an image using a reverse image search. TinEye used to be the best service available, but Google's Search By Image usually produces the better results. It is wise to check both, however. There have been a few instances where TinEye has been able to find an image for me when Google did not.
As a rule of thumb, I would never use an image if am unsure of its distribution license. This is especially true if you intend to monetize your web app.