Curiously worm gear mechanism 3D model

I am very pleased with the image(s) I purchased but I would have preferred to have the exact terms of use specified in a file within the downloaded package. As it is, the conditions under which it remains "royalty free" are not entirely clear to me. (I just want the image to use as an illustration in a not-for-profit context, but it is not clear whether I am free to modify it in any way. An appropriate Creative Commons licence would have been extremely helpful.)