You can utilize FFMPEG to grab a screenshot of your live stream as follows:
ffmpeg -i rtmp://[your-wowza-ip]:1935/[app-name]/[streamName] -f image2 -vframes 1 snapshot.png
Furthermore you can get successive ongoing screenshots by using:
ffmpeg -i rtmp://[your-wowza-ip]:1935/[app-name]/[streamName] -f image2 -vf fps=fps=1 snapshot%d.png