This question has been answered by lettis. See the first response.
sources - To manage Media Sources and Media Source Types. source_delete - To delete a Media Source. source_edit - To edit a Media Source. source_save - To create or save a Media Source. source_view - To view and list Media Sources. view_tv - To view any Template Variables. edit_tv - To edit any Template Variables. save_tv - To save any Template Variables.
if you want to try with my policy, it is attached here. Exported from a revo 2.2.8, but i think it doesn't matter.Thanks a lot. It seems like you allowed almost everything. Anyway do you have any idea which value could be responsible for allowing image-tvs to edit?

I assume it is working now ?!Not really. I don´t want to giv all permissions to all persons. Therefore it ist just ok for the moment until I figured out what permissions are needed to edit the values of an image-tv.