Or upload it as an attachement.
Click the "manage attachements" button under the "additional options" menu when you're writing a new post.
You can upload pictures from your computer there.
dougal - (see [avatar] man with guitar - is that you???)
photobucket is one of the handiest places I know to store pictures on the internet where you can access them for use within forums. That's where my avatar and my personal "signature" pictures are stored.
One of the programs you will need to use a picture for an avatar is a "re-sizer" program to get your avatar to meet the smaller sizes used by forums. You will be able to find a free resizer on the internet if you just look - the one I used is called "Autosizer".