That's a pretty informative photo actually.
One thing I've noticed is that people tend to see cosplayers, so getting an accurate at-a-glance estimate is somewhat difficult. When I looked at that picture, it initially looked like about 4/5 of the people were cosplaying.
I also spend most of the con in the game room, which tends to be one of the primary attractions for non-cosplayers, so I had a few factors I had to estimate around. I think that probably pictures of the reg line (like the one Hazuza linked to) are the best way to get an accurate estimate. Though, as stated, some cosplays can be hard to recognize as cosplays, and sometimes I'll get mistaken for cosplaying because I tend to wear extravagant or unusual clothing at cons.