I think it really depends on how you're using references. Are you looking at beetles to try to understand proportions, features, how to draw your own? Or are you wanting to copy the image or aspects of it?
Not all stock images have the same usage rights and many are licensed so that you can only sell x quantity of work produced with them. The same goes with font usage.
I would just search for "royalty free" stock images or under search tools, check the usage box dialog and then double check the website the images are hosted on. Sometimes the person who owns the image will require a link back or credit, other times you can purchase usage rights, or use them for free without credit. There's a lot if variation, so I'd just verify the usage perimeters on the actual website.