You can go to http://www.google.com
and click the link by the search box that says "images" then type a word or two in the search box for what kind of picture you are looking for. Look through the results, when you find the one you like, right click and "Save Image As..." and save the image to your desktop (easier to find when it comes to uploading). Come here to the forums, log in, go to "User CP" and to "Edit Avatar" then click the "browse" button next to the empty box where it says "upload your own image" and go to your desktop in the explorer window that pops up and select your file. Then click "Open" in that box and "Save Changes" at the bottom of the page. You should see your new avatar next time you look at one of your posts or go to the "Edit Avatar" screen.
Hope that helps!
If you have any questions, let me know!