Photo-based Sketch

I downloaded an image some time ago from a site I can't recall, and I think it was for a brand lookbook. The clothing wasn't so remarkable in my mind, but I did like the stance of one of the male models. I'm not sure what to do about pulling references like that - when it's an informal sketch that I just do for practice as I do here, do I need to reference the site and its photographer? I'm not sure what constitutes fair use in this case. In any case, I don't recall the site it came from, so that's the background info, and this is the sketch. It's a rougher fashion-y style that I've come to be comfortable doing - which is strange for me because I'm not sure where it comes from, other than wanting to use certain tools. Perhaps it's the genesis of my own style? Hunh. Fancy that.