Personally I do the same sometimes. There is a section on DeviantART called Resources and Stock Images; and if I need inspiration or help with anatomy on a pose I will usually search through the images since I figure it is safe to assume that they are all posted there with the intent to assist other artists.

Normally when I do this I make a note in y description of the final image that I did use references to help me with the image. I figure that as long as you give credit it is acceptable. And of coarse not tracing it line for line and adding your own personal flare to it.

Now if a commissioner had issue with me referencing another image then I'm not quite sure what I would do. Either redraw or refund them I guess; I'm interested to see what others may have to say on the matter.