Resize Generic Camera Picture

Hello,

I need help with my Home Assistant installation please. I am trying to connect an older camera system (webgate) to my installation.
Unfortunately this does not offer rtsp or onvif streams, just proprietary software.
But it is possible to retrieve a .jpg still image via http. The Generic Camera Integration works with this URL.
However there is a problem: the .jpg is delivered in the wrong aspect ratio from the DVR (704x240 - not 16:9).
Now I want to convert this image to the correct format. I have already tried the Camera Proxy Integration, which unfortunately does not allow to resize the image regardless of the aspect ratio.
I also tried the ffmpeg integration with the corresponding video filter option. The image looks correct with it, but unfortunately it only works for one frame at a time until it breaks.

camera:
  - platform: ffmpeg
    input: -stream_loop -1 -r 1 -i 'http://192.168.1.30/jview/ch1.jpg'
    extra_arguments: -vf "scale=704:396"
    name: door_test

Is there a better way to display the image correctly or did I make a mistake?

Many greetings