How far has image recognition software gone in development?
My friend, a photographer who mostly works in stock photo photography, has a bunch of images. Is there some software available which offers the options to "recognize" the number of people in the photos and to sort them according to that (he had some other wishes as well ... but I can't remember those now
I myself, do not know anything on the topic so it's hard to be specific on what I'm looking for. Or whether does something like that actually exists.
I'd appreciate any advice on the subject though.
I don't know of any software that can do what you want automatically, but I know from experience that Adobe Bridge  lets you tag your photos making them easier to find (think tags like "2 people", "trees", "park", etc).
Just today, I even read about a research project at Tsinghua University, Beijing,  that is able to take a labeled sketch and create a compiled photograph! Check out the video at the link. http://www.adobe.com/products/creativesuite/bridge
iPhoto lets you tag the faces in the photos, and it'll recognize and suggest the person among all the photos and let you confirm.
Also you can create smart album with options like "show all photos with faces of both Mary and Gary".
Check out the iPhoto introduction from here . http://www.apple.com/ilife/iphoto/#overlay-organize
It definitely exists. To answer you question "How far has image recognition software gone in development?": Check out
. One of the leading conferences on that regard.
I've coincidentally stumbled upon it reading this article  (warning: it really did blow my mind ;-)
Also related, TinEye , a reverse image search engine, is in beta. Given an image it is supposed to find others exactly or somewhat like it. http://www.tineye.com/
It's a very difficult problem especially when you have varied requirements. One of the best ways that images get tagged and analyzed is by turning the task into games. This sort of research has been put forth Luis von Ahn (the same guy who created captchas) at Carnegie Mellon who developed the ESP Game  and Google Image Labeler .
Personally, I've used some of the same technique with my own photos. Except that I can't really generate a large enough community to create a clone of those games. Instead I've put up a small website and hooked it up to Amazon Mechanical Turk where I had people provide suggestions for tags and other attributes. It's a little bit of work, but hey, for $0.01/photo I was able to get some pretty good information about my photos from real people. http://www.gwap.com/gwap/gamesPreview/espgame/