I would normally change this myself, but Arthur's page is so huge that I feel I should post this first (and I don't know what mission that image came from).
I actually really like Arthur's leading image, it's well-shot and represents his character well. However, I feel that it is heavily dragged down by the fact that it uses a clean-shaven, overweight Arthur.
I personally believe that this image should be replaced with the same shot, but with Arthur's default appearance (level 3 beard and hair, perfect weight, "The Gunslinger" outfit).
Certainly, a character's default appearance is what an image on the wiki should show. As far as Arthur's photo goes, the only noticeable thing is that he is virtually clean shaven, although I wouldn’t say that that’s a massive a problem given that everything else is right. A few months ago, I took some photos of Arthur myself, but as I always wear a bandolier and never the neckerchief that Arthur wears, it wasn’t quite the default outfit. Furthermore, getting the lighting right in the photo of him in Valentine sheriff's office proved quite difficult.
While I am satisfied with the current photo, changing it to what you suggest would be fine, but only if the graphics, lighting, etc are just as good.