Questions about Using Reference Images

Hi,

My question is two-fold: first, about acquiring free images online, and second, what should be done when using such images as references. By references, I mean setting them up as background images and using them as guides to make 3D models.

1) If we want to get an image from the internet for free (regardless of the use), we must make sure that it's under the correct license, right? Like CC-BY? It's easy to just right-click a thumbnail on Google Images and "Save As...". But isn't this wrong? Would it violate copyrights? I want to know the right way (and the "rules") to just *get* free images online. (I know Flikr has a lot of images with various CC licenses). 

2) If we use such images as references to make 3D models, do we have to follow the license that we got it under? The images themselves aren't being used directly, so do we still have to provide attribution for a CC-BY? If the license doesn't allow for "derivative work" (the meaning of which I'm not entirely sure), does making 3D models from these images count as derivative work? If the license doesn't allow for commercial use, can the 3D model (or whatever it's used in like a movie or a game) still be used in a profitable way (either directly by selling the model/movie/game or in some other work for profit)? Does the model (or whatever it's used in) also have to have an "identical" license if the reference image is licensed as CC-SA? I'm just a little confused because the images aren't directly being used. 

Finally, what if the terms or licenses aren't very clear? For example, if one wants to use a screenshot (2D Top View) from a mapping software like Google Earth to use as a reference to make a 3D model of a location, what are the regulations behind this? I realize it might be a question that should be asked to lawyers, but just wondering if anyone has any input...

Thanks