Why am I not always allowed the option to crop a picture I import?

Once in a while I'll import a "photo" -- in this instance, it's a screen shot of a book page -- and I am not given the option to crop.  By that I mean the "crop" option does not "light up" or offer itself as an option.  I want to crop out all the computer screen stuff shown on the screen shot and just end up with the shot of the book page.