The pictures appearing on your BookingButton are directly pulled from your Booking.com Extranet. In order to change them you need to do so directly on the Extranet under ‘Photos’ in the tab ‘Property’. A step-by-step instruction for changing or rearranging pictures can be found in the Booking.com help center here.
If you change your picture in the Extranet, those changes will also be reflected on Booking.com additionally to your BookingButton. Please note, that it can take up to 24hrs for the changes to appear.