The question of reference materials often comes up in class and in online workshops. How do you use it? What is acceptable use? Are you violating copyright? How much can you copy something? There are so many questions and so many answers.
Remember, that it is called "reference" and not "copyance"... You refer to the image and not copy it.
I figured I could make up my own word like football announcers who seem to all now use the word "trickeration". WTF! That can be the subject of another rant.
Here is an example of how I use reference.
I don't know where or when I found this big-nose version or Robert De Niro. I have a hard drive of all sorts of subjects from all over the place. Someday I will get them organized so I can actually find things when I need them. I don't know why this image caught my eye when I was painting the Crypt Keeper but it did. I referenced the image while painting the face of my character. The mole in particular showed up in this early version. My crypt keeper may even look a bit like Robert's long lost cousin. Only you can tell.
At any rate, you can see things from the original image that have been incorporated into my painting but it is by no means a copy.
In my opinion and in my classes, this is how reference materials should be used. Especially reference materials that you have not photographed yourself.