I recall trying web sharing in my system prefs once before but I was not able to access the URL when cut and paste to by browser. Im presuming this is being blocked by our network at some point. Its no big deal
Enabling Web sharing like this will open your mac's ability to publish web pages. However, there is more to it than that. Being on a local area network often mean that the IP address allocated to your mac will ONLY WORK from within that same network. If they are any competent, your network administrator has shielded you from the internet while exposing just a few machines to the outside world.
You probably know this but when, from your mac, you ask to see a webpage, your mac sends a request to a machine that has both accesses to the outside and inside world. That machine will get the webpage for you and send it back to you according to your local IP. But for this to work, in a basic situation, you have to initiate the transaction.
Of course there are way to circumvent that (on your side) with the help of your Admin by "punching a hole" into your Firewall. Doing this your Admin will assign a PORT to you and redirect any traffic made to that port to your machine. I have to tell you though that most administrator are squeamish about doing this as they lose the "integrity" of their Firewall. In essence, any malicious intents going through "your" port would have to be dealt with by your machine or else they will have access to your whole network.
That said, it is much simpler to register for a free web hosting account where you could upload your avatar. Chances are, if you have access to the Internet from your home, it is very likely that your basic package from your service provider includes a small web presence area. You could also ask your admin to put it on a webserver that has access to the Internet.
