Now I understand.
I thought the plugin would move directly into the box "Featured image".
Is there a way of effecting exchanges directly in that box?
Thus no change in themes and your plugin would be required would be more intuitive and more used.
Another thing you could implement in your plugin would upload using drag and drop, as there is this plugin: http://wordpress.org/extend/plugins/drag-drop-featured-image/
Anyway I am very grateful for your help!
Hugs!