I have a few commissioned characters that I paid an artist to draw, and while I wouldn't mind someone using their image as a general reference. His beard looks like this, or her hair has this style. I don't think anyone should try to pass the whole image as their own. You should at least write a general description and if you feel a certain image helps portray the character you created sure. Go ahead, just don't claim the artist's work as your own. Typically I've almost always tried to depict my characters in words, and while sometimes I need to research for a more accurate description as I find that I lack the words I seek to describe them I try to keep my use of images to a minimum. For things like outfits or specific styles that are generally hard to explain.