You'll have to purchase premium from the store to be able to fully modify your account, if you have, you have four options other than that of an uploaded image: A duck, Robbie Rotten, a different duck, and the default icon of another duck. If you use any of the predefined icons, you'll get extra FH Points for your account.
An image that you upload must be at the irregular size of 243 x 279 in JPEG, otherwise it won't work.
The music in Feral Heart is not my jam, can I change it?