Bit shite and needs tweaks but:
$ cat stream.sh
ffmpeg -loop 1 -i white.png -acodec copy -shortest \
-c:v libx264 -preset fast -pix_fmt yuv420p \
-s 1280x800 -c:a aac -b:a 160k -ar 44100 -threads 0 \
-f flv "rtmp://live-lhr.twitch.tv/app/MYSTREAMKEY"
white.png just being a square png