hey,
As far as I know there is no easy way to get an image from a stream!
So here is my way of getting an image from a stream (worked on my ubuntu server)
First of all use the package livestreamer to download a part ot the stream
livestreamer --output stream.mp4 --twitch-oauth-token [token] --hls-timeout 1 --stream-types hls -f Twitch best &
Use the hls-timeout to automatically stop after a few seconds
Wait for a few seconds to let livestreamer stop
sleep 5
Then use the package avconv to extract an image from the mp4 file
avconv -i stream.mp4 -frames:v 1 -qscale 1 -aq 1 stream.png
Youre image is now saved as stream.png
I hope this helps you a bit 
Greetings
Marci4