Your best bet is to find an image that's not too big. Preferrably the size of a thumbnail image from a website. They have a limit to the image size you can use for your avatar here, and I believe it's about 175x175. Once you find the image, save it on your computer. Use the free image hosting site "Imageshack"-
http://www.imageshack.us/
and upload the image there. It will turn that picture into a URL to be viewed on the web. Take that URL and put it in your profile at the bottom where the avatar is entered. Imageshack is great for uploading images to the web to share in this board, as well as other uses. For instance, the image below where I'm asking people to vote for Charlize Theron for the Babe Hall of Fame, here's the URL for that.
http://img131.imageshack.us/img131/7024 ... ge22rk.jpg (and a yes vote would be appreciated!)
You can put images in your signature, your avatar, and in your post that you upload to Imageshack. If you have further trouble, you could just post the image here and I'll upload it for you. Make sure the image isn't too big though. If it is, the admins will remove it, or ask you to remove it.