Disclaimer : I'm part of the same company as the Genymotion team.
This feature is included in the product. It is one of the paid feature of the screencast widget. Look at the pricing page here.
Two ways to access it:
- pay for the pro or indie licence
- use the trial version, it offers you the indie features. Be careful, there is only one trial day left :-/
Once your VM is started, open the screencast widget
Then take a picture with the dedicated button
UPDATE: You have bellow another ways to take a screenshot using Android Device Monitor or the command line