You cannot copyright a pose itself. Evryone does that exact same pose at anytime of the day. Well depends, some people are more flexible than others.
But you get my point xD
To me you can use an image without consent just for the pose if its in terms of practicing. Like say, you want to practice anatomy and you need models etc so you use photos you see online, or posemaniacs, what have you. If its a more serious piece that you will publicize, and. You used refs, it is proper to state such things. Especially if its stock owned by other artists.
Its thanking them for their work in a way.
But more to the point, nothing beats taking your own pictures and merging as well. You can pick this bodypose with this hand gesture, etc.
We cant really help drawing similar looking poses, if you draw a cofee scene and someone next to you draws the same, youre bound to have the same people sitting etc.
In lame mans terms, if the reference was not yours to begin with, you just have to credit them. Free or Not.
If you draw from real life/make your own refs, its not a problem, its YOU who observed it from natural things, not another persons work, ya know?