Firstly, you need to ensure your app has the 'My Account' feature before you are able to change this image.
You are able to change the image that users see when they access their account through the app via the CMS.
This is done by clicking the 'My Profile' tab and amending the corresponding Content Block that is used for this banner. You can find this Content Block in the sections for Registered Users.
For more information on Content Blocks, read here.