Your profile picture in Mad Skills Motocross 2 is the same as your connected social media profile picture.
If you click on the globe icon in the settings menu you’ll see multiple social media account options.
Once you log in with a social media account the profile picture from that account will become an option on the right side. You can hit the arrows to scroll through and select which picture you want. For instance, if you are connected with Game Center and Twitter you can choose which profile picture to display, as seen in the images below.
Note that profile pictures for Google Play are not supported at the moment.