To add a watermark to your actual video files, please add the following to your ffmpeg command lines for desktop and mobile.
Top left-vf "movie=/full/linux/path/to/watermark.png [watermark]; [in][watermark] overlay=10:10 [out]"
Top right
-vf "movie=/full/linux/path/to/watermark.png [watermark]; [in][watermark] overlay=main_w-overlay_w-10:10 [out]"
Bottom left-vf "movie=/full/linux/path/to/watermark.png [watermark]; [in][watermark] overlay=10:main_h-overlay_h-10 [out]"
Bottom right-vf "movie=/full/linux/path/to/watermark.png [watermark]; [in][watermark] overlay=main_w-overlay_w-10:main_h-overlay_h-10 [out]"
Center
-vf "movie=/full/linux/path/to/watermark.png [watermark]; [in][watermark] overlay=main_w/2-overlay_w/2:main_h/2-overlay_h/2 [out]"
watermark.png should be a transparent PNG file with your site's logo.