Well first you have to upload a suitable image to the SimV server (no more than 10kb and 100 pixels wide), then you just go into profile and edit you avatar.

You're going to want to go here:
http://www.simviation.com/yabbupload.html

Upload your image, make sure you read the instructions before you do! (believe me I've made that mistake once before!!!)
Once it's uploaded there's going to be a code, it'll look like [img ]http://simviation.com/.....etc.jpeg[/ img]
Copy that code exactly as it is, exclude the spaces because I put them there so you can see what I'm talking about (they won't be there)
Go to your profile, hit the modify button next to your name, punch in your password, go to the options section and paste your [img] code into the signature box and hit the change profile button at the bottom of that page.

Hope that helps