I too had this problem on iPhone. The solution is to log into BYC using safari on your phone rather than the app. Then click desktop version at the bottom of the page. Click on your avatar. Select add photos or edit photos then you can select a different photo or upload a new one.
Scroll to the bottom of the page on your phone. It will show two buttons Desktop/Mobile click on desktop. You will have access to changing the picture. You can always go back to the mobile view when finished.