You'll need to have the picture uploaded to a photosharing site, I use photobucket.
You'll have to create an account (it's free) and upload the picture there. There's a green "upload" button on the page, once you have signed in. Click on upload and find the pic in your documents, double click on it.
Once it's in your album, go to the pic you want, click on it. Underneath it you'll see a few links for the picture. Click on the one that reads "direct link." (Just click on it, once you do that, it'll change to "copied.")
Now, go to your profile on here, near the bottom, you'll see some blank spaces for avatar and signature images.
Right click in the avatar image and click on paste. It'll paste the link for the pic from photobucket there.
Click save changes and you're done.
Hope I didn't make it too confusing, just trying to take you through it step by step. Also, like the other posters said, make sure it's an appropriate picture.
This post was edited on 12/10 at 4:57 pm