Once you've found the picture you like, right-click on it and choose
Save Picture As... or
Save Image As... and save it to your computer.
If it is larger than 180x250 pixels, it will need to be made smaller*.
Then click the
Browse... button at the bottom of the
edit avatar page, find your picture, and click Save Changes.
*If you're not sure how to do this, tell us the internet page where the picture is and we can do it all for you.