"Artists Beware"

So I wanted to start off saying, I've seen some comments vary across the internet on pose copyright/trademark and usage and it seems a lot of people are confused about someone's rights ect. I am sort of in that latter. 

I went searching AB and google and couldn't find any good citations of legal standing on if poses and "image concepts" are legally bound as are say trademarked characters or artists legal rights to their art and came up dry on both. 
Long story short, I had a commissioner today note me on FA stating someone noticed the pose reference picture that we used on one of their commissions from me. Which would be fine by all means, except this person went on to state I was ripping their friend's art off and it was illegal to use their friends picture as a pose reference and concept for their commission and was pointing out that they would be infuriated if someone did that to them. (Commissioner feigned not knowing about anything on the reference for fear of a drama storm, which is fine by me. I also didn't reply to this person because I wanted to be 110% sure in what I tell them is backed up law and not also start unnecessary drama.) I've also added a link back to the original image we used for the commission pose reference in my fa gallery.

In short, my questions would be, is it ok to reference other artist's art for pose reference whether in personal art or commissioned art if the commissioner requests it? Should we link back to the picture(s) we referenced just the pose from? Are poses and or "concepts of the image" legal to use as reference or do we need to ask the artists permission first?

(Clarification of "concept of image" I'm speaking of would be, a person skate boarding, an android women kissing someone, two characters in a staring look, ect as a few examples off the top of my head right now.)

Thanks for your time.

*EDIT* I wanted to point out, that I think everyone has been looking at the wrong submission. I have an idea of which one you are looking at and have credited back to the original reference. And will be doing so from now on. I appreciate all the feed back and honestly had no idea had to reference back to what pictures I was using for poses. I shall do this from now on. I'll also be adding in my tos about commissioners and referencing poses ect. The submission I am talking about was done middle I think of last year so you would have to do some digging to find that one. And I've already referenced back to that pose on it. 
I would rather not post a link to the image in question in public for fear of naming my commissioner without their permission. If anyone would like to see the two images, please message me privately and I can link to them for you. Thanks again for your help!