Unfortunately, you will not be able to put an image or picture in the game directly. The only way for you to put an image or picture in your avatar is by linking your game account to your Facebook profile. Upon linking, the game will automatically adapt your current Facebook profile picture.
If you are unaware on how you can link your Slots Reel Frontier account to Facebook, then please do follow the link Here.
Reminder: It may take some time to fully update or port your Facebook Name and Profile picture to your Slots Reel Frontier account. Please do wait it out for awhile for this to happens to your account.