Here’s a quick and dirty way to grab a thumbnail from a Wowza application:

rtmpdump -v -B 0.01 -r rtmp://wowza.server/application/stream -o temp.flv
ffmpeg -i temp.flv -vframes 1 -s qvga /var/www/frame.jpg

There’s also a way to do it with a Wowza module, but it’s considerably more complex and not for the faint of Java.

Caveat: This likely won’t work if you have hotlink denial turned on on your stream.