You can override the 'default_cover.png', by placing an image of your choosing with that name in the calibre configuration resources/images folder.
To get to the calibre configuration folder press the
Open calibre configuration directory button in
Preferences->Miscellaneous. I suspect you'll need to restart calibre for the change to take effect.
See User Manual ==>>
Overriding icons, templates, et cetera
Basically everything in the install
resources sub-directory can be overridden by a corresponding entry in the configuration
resources sub-directory.
BR