If you can describe how a person looks quite well, then describe them.
Otherwise a picture is the safer route. Personally I enjoy pictures a bit more, since I tend to gloss over details and instead focus on dialog and the plot.
There are some artists on the site. If you're REALLY lucky, and very nice, they may draw a character or two for you. Or you could always try to outsource the drawing job to another artwork site.
As long as you link the picture to it's source and give credit to the artists, they are usually fine with you using their pictures. If you're unsure, message them on whatever site you see it on, and ask permission to use it (assuming they originally created it).