When someone posts photo reference from another website, if I know the website I always credit them. If I do not see the watermark I just move for the sake of time.
Maybe in your particular case David you can complete a drawing drawn from one of the photo reference here. That would be a good option. Now I completely understand that you may not connect with my photos.
In my opinion crediting the website is basically me just giving them free advertising. I think they would like that.