I think you have to reach the "User" level to be able to add a custom picture to their profile.
You can reach that level fairly fast If you start entering bills every day.
If you check my profile you will see it took me less than a month.
You can enter you own bills that get from the bank or in change , It is a good idea to write the website on with a Sharpie, of preferably get your own stamp.
You can also just enter unmarked bills (called Stealths) although your chance of getting a hit are next to Nil.