Lately I've got into the habit of taking photos of exhibits in museums and basing pictures on those. This is not how I was taught to behave in art college, but quite honestly if something catches my eye I'd rather draw it this way than get in everybody else's way while trying to draw it on site.
I actually got a lecture from a couple of teachers at Epsom School of Art and Design about the evils of using photos as reference, and to a certain extent I got what they were saying; if there's something in a photo that is a little badly-defined or in any way obscured in the shot, it's impossible to remedy it except by returning to the scene of the picture. You can't, as my teacher put it, "see around corners".
But, as with a lot of what I heard at art college, their angle was an opinion. There are plenty of good artists and illustrators out there who clearly use photos - just the other day I came across this article about Normal Rockwell's technique: http://www.beautifuldecay.com/2014/08/22/norman-rockwells-reference-photos-iconic-paintings-revealed/