Am I risking copyright infringement?

I'm a beginner graphic designer working for a non-profit org, in the process of illustrating a book. I'm making pictures, many of which use parts of images found mostly through Google Images and I’m becoming aware and more and more concerned about copyright infringement. One picture, for example is of a desert background, filled with a couple lions, snakes, scorpions, vultures, a hyena, all taken (downloaded and cut out) from separate photos, and composed in Photoshop. Everything has color, lighting and blending effects and the picture as a whole has a very strong message unrelated to any purposes of the original images. I'm wondering if our uses would mostly be considered fair use because of the fairly insignificant portion and role that the separate animals play individually in the overall picture; the more factual as opposed to fictional works (plain photographs of the animals, or of the desert) being used, and the much more fictional/creative work being created with it's unrelated, non-competitive purpose and message. It's a philosophical, educational book and we do want to be able to sell it. Is it always infringement as soon as I download any copyrighted image and put it or a part of it directly into my project, to change it from there or not? Or if that passes, is it infringement to change or edit that image, like by putting different paws or a different head on a lion or putting spots on it or something? Any answers or help would be so greatly appreciated. Thank you.