there is an option to ‘Add new image’ and to ‘randomize suggested headers’
Why not just add one tiny feature to ‘Remove a header’.
Now you have all options to create your own header randomizer.
Maybe you need to call the other button ‘randomize headers’ then.
I tried the following:
– ftp a new header lets call it test.jpg (1400×320) to ../wp-content/themes/graphene/images/headers
and also test.thumb.jpg (364×83) also to ../wp-content/themes/graphene/images/headers
but the theme did not pick the new one up. So there must be something else