In januari I was completely stuck, so I contacted Kevin Godell. Got immediate help from Kevin, so would like to thank him for his time!!!!!! After some research, he came up with following ffmpeg command:
-f mjpeg -i pipe:0 -vf scale=320:240 -f image2pipe pipe:1
When you use this command in Node-RED's daemon node, it is now possible to pass images via memory (stdin stream pipe:0) to ffmpeg (since we (ab)use an mjpeg stream to be able to process an infinite series of images). And the resized image is returned back to Node-RED as an output message (stdout stream pipe:1):
[{"id":"122804f3.f8b7eb","type":"daemon","z":"50df184.124f2e8","name":"Resize image","command":"/home/pi/FFmpeg/ffmpeg","args":"-f mjpeg -i pipe:0 -vf scale=320:240 -f image2pipe pipe:1","autorun":true,"cr":false,"redo":true,"op":"buffer","closer":"SIGTERM","x":560,"y":320,"wires":[["98ce6e9f.81789"],[],[]]},{"id":"631c9e62.c5bf1","type":"inject","z":"50df184.124f2e8","name":"","topic":"","payload":"","payloadType":"date","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":180,"y":340,"wires":[["afda2c1f.ec754"]]},{"id":"dfde7d7e.a8b0b","type":"image","z":"50df184.124f2e8","name":"","width":200,"x":980,"y":300,"wires":[]},{"id":"afda2c1f.ec754","type":"http request","z":"50df184.124f2e8","name":"","method":"GET","ret":"bin","paytoqs":false,"url":"https://static.independent.co.uk/s3fs-public/thumbnails/image/2017/09/12/11/naturo-monkey-selfie.jpg?w968","tls":"","proxy":"","authType":"basic","x":340,"y":340,"wires":[["122804f3.f8b7eb","c886e69d.47aa98"]]},{"id":"93ccc4c3.6f45c8","type":"image","z":"50df184.124f2e8","name":"","width":200,"x":720,"y":400,"wires":[]},{"id":"c886e69d.47aa98","type":"image-info","z":"50df184.124f2e8","name":"","x":530,"y":400,"wires":[["93ccc4c3.6f45c8"]]},{"id":"98ce6e9f.81789","type":"image-info","z":"50df184.124f2e8","name":"","x":790,"y":300,"wires":[["dfde7d7e.a8b0b"]]},{"id":"853bae8d.c380d","type":"comment","z":"50df184.124f2e8","name":"Image processing without spawn but with delay","info":"-f mjpeg -i pipe:0 -vf scale=320:240 -f image2pipe pipe:1","x":660,"y":260,"wires":[]}]
Advantages:
- All data is passed via memory, and no temporary files (or disc IO) is needed.
- A single daemon ffmpeg needs to be spawned that processes all images. I.e. it is not necessary to spawn a new ffmpeg process for every image, like e.g. the ffmpeg-stream package.
Disadvantage: there is a single image delay 
When you insert an image nothing happens. As soon as you inject the second image, the first image will arrive on the output. And so on ... Not 100% sure what the problem is, but after the first image is inserted I get only this output (on stderr and adding -loglevel debug):
[mjpeg @ 0x7fffcd526700] Before avformat_find_stream_info() pos: 0 bytes read:32768 seeks:0 nb_streams:1
This is similar to the issue I had before: since we (ab)use an mjpeg stream, ffmeg wants to collect information about that stream. And I think that causes the delay. But I'm not sure...
Unless somebody else has some usefull tips, I will have to stop with this mechanism 
Would be a pitty, since we are almost there. And it would have been an awesome addition for Node-RED... P.S. I'm not referring to the image resizing, because that is only a simple test that you can also achieve with node-red-contrib-image-tools in pure Javascript ...
