What you should do is to set all the buttons that you want to see (in settings.py). Something like:
TINYMCE_DEFAULT_CONFIG = {
'plugins': "fullscreen,paste",
'theme': "advanced",
'theme_advanced_buttons1' : "bold,underline,italic,strikethrough,bullist,numlist,"\
"separator,undo,redo,separator,link,unlink,image"\
",separator,cleanup,code,removeformat,charmap,"\
"fullscreen,pasteword,formatselect",
'theme_advanced_buttons2' : "",
'theme_advanced_buttons3' : "",
'theme_advanced_toolbar_location': "top",
'width' : '800px',
}
Pay attention to the image button that you need.
I use it together with formfield_overrides in admin.py like you wrote.
Hope it helps, Alex A.