Once you reach the Pro level, select the race "170 mph in Heavy Traffic". When it starts, go to the opposite side of the freeway so you are going with traffic. Once you finish, instead of pressing A, keep doing the same race until you getting tired of making easy money!
It's easy! Make sure you're signed in, then go to your profile by clicking on the username you used to register (top right of the page). One you get there, look for a link that says "Edit" just underneath the grey and white default avatar. You can then upload a picture for your avatar as many times as you like.