If you look in the Banner module at the Banner class, you'll see the "Banner Image" is actually the BannerContent field and using a form control of "Banner Content". The form control manages the controls within that image selector. So you'd have to modify this out of the box file OR (my suggestion), clone the form control and make your modifications to that form control. Then go into the class definition and use the newly modified form control you created.
~/CMSModules/BannerManagement/FormControls/BannerContentEdit.ascx