The above comic is from the Joy of Tech blog, and I got it from Mashable. They've covered generally just about every type of avatar out there - in a mostly tongue-in-cheek manner.
You probably know by now that you're going to need an "avatar," or profile picture for just about every social networking site on the web. I personally recommend a good headshot for most business-related social networking, but many people are uncomfortable posting a photo of themselves, either because they don't like the way they look, or just because it feels too "vain," to self-promote. Others have security or privacy concerns with posting their own profiles.
Whether you choose to post a photo of yourself, or some other type of graphic, here are some things to keep in mind when choosing a social networking avatar:
If you have security or privacy concerns with posting your own photo, please DON'T substitute your own picture with one of your kids or pets. They are more vulnerable and make easier targets than you do.
Remember that your avatar is the way people will quickly recognize you and find your posts on the various sites. DO choose something fairly unique to you, easily recognizable. DON'T "change it up" every couple of weeks. This just confuses people.
DO use something that represents you or your business. DON'T use a celebrity or political figure. This is technically illegal (it's a type of impersonation, even though it's seldom prosecuted because most people realize this is either done by fans or as satire). It's also a common tactic used by spammers - who aren't planning to stick around under that profile very long, and are too lazy to come up with something original. Finally, it shows lack of personal self esteem - not a good image to project if you're networking for business purposes.
DO be a square. Many of the networks actually require you to crop your image to a square one, and a square one will work even on the networks that allow other shapes. Having a good square image on-hand will save you time and frustration when you're signing up for a new site.
Finally, although this should be the most obvious, I'm amazed at how often I see this being done - DON'T use the site default for your avatar. This screams, "I'm new to the Internet," which, in 2011 is not an image you want to project. Take a few minutes to develop an avatar that works for you, and that you would like to use for a long time, and then use it. Just do it. :-)