On most message boards you normally can post an avatar from your computer but for pictures you normally need a URL on the internet. So if you see a picture on the internet all you need is it's location. If it's a personal picture then it's best to download them onto a website, I use
ImageShack® - Online Photo Hosting
For example, here is a photo I took of our kitchen and then downloaded it to 'Imageshack' and then pasted the URL for that photo onto this message board here in my post. You can see the URL I used to post the photo at the bottom.
[IMG]

By [URL=http://profile.imageshack.us/user/howardbeale]