ffmpeg batch apply awtermark to thumbnails

Hello,

I'm trying to batch apply the ffmpeg watermark to existing thumbnails but I can't figure out a way to do so.
I already managed to call a controller via a script so the problem is not there.
The issue is that whether you select "Add (or Remove) watermark from all existing movie thumbnails", the url that is visible in apache log is the same
Any idea on how this could be done would be appreciated.

Nono