In this case my onvif nodes will also not work. Indeed my nodes also fetch the snapshot image via a http request, since the Onvif protocol returns the http url...
You should use the node-red-contrib-ffmpeg-spawn node. Since it is also not available on npm yet, you should install it with the commands on the readme page. And of course ffmpeg should also be installed.
You should have a look at this discussion, which might be a good starting point.
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