"Artists Beware"

As with other comments: if it's a very noticable difference, bring up the difference with the artist when you recive the image, and ask if they can work on it some more until it's more like their sample images. If this were a real-media sketch though, sometimes you have to just "suck it up", know that you got a "bad" sketch, but let it go anyway and move on. Thankfully something like that only happens about 5% of the time, but if you commission things long enough, eventually you'll get a "stinker" from someone. Just as equally, you can have someone do something so over-the-top fantastic, you're amazed at what they did, so just balance out the "good" with the "bad".