I believe that putting people's pictures on the web is a bad idea.
Placing pictures on the web has very little use. It doesn't help deal with email faster, or to find the right web page easier (it actually slows things down.) It doesn't contribute to scientific progress.
Some have argued that you can gain a competitive edge when you attend research conferences because you will know all the big-name researchers on sight. Bologna! This position argues in favor of putting everyone else's picture on the network except your own. (After all the big-name researchers don't have time to cruise the web to memorize your face, so the only people who will know you by sight because of your web picture are people who you probably don't want to know!)
Some argue that there is a social contract: If you want everyone else's picture on the web, then you should put your own on the web. Well, I don't want everyone's picture on the web, so I don't put mine on.
There is actually some harm to putting your picture on the web. You can become the target of various forms of harassment and discrimination. For example, in early December in the Boston Glob, there was an article about "Cruising the Web for Babes" by two Glob reporters who were acting like immature morons. The thesis of the article was that they could find the email addresses of hundreds of good looking women by cruising the web sites of the ivy league colleges (especially the women's colleges.) What a great idea! Not! Fortunately, they didn't have much success.
Some have argued that women and minorities especially should put their pictures on the web so that other women and minorities will see examples of people like themselves who are doing whatever it is that they are doing. It is argued that these examples of success are needed to encourange, for example, women to study electrical engineering. This argument has some merit. Each of us must decide for ourselves how to factor this issue into the decision of whether to put our pictures on the net.
I don't put my picture on the web.