I use Screenshot Captor to make screenshots of applications and mark issues (bugs) in there.
The applications run in full-screen size, so the screenshot has the same size than my screen resolution.
If I want to see the complete screen shot (to see if I have forgotten to mark some issues) I must reduce the zoom to 85%.
At this zoom it is hard to identify issues and to measure font sizes or color contrasts.
The best solution would be to see the screenshot in 100% zoom.
I suggest you allow the user to drag the Thumbnail Panel and the Toolbar (the bar with the buttons and the element properties like transparency or shadow) out of their positions, so they are floating on the screen.
This way I can move them to the second monitor.
Of cause you must allow to dock them again at their original position.
The Zoom-Slider could either be moved to the top (left to the checkbox "AutoFit") or in the statusbar.
Or you attach the zoom slider at the toolbar.
In the preferences the user can decide if the elements should be floating and where the zoom slider should be positioned.
This feature would save me many mouse clicks (zooming).
Please consider this suggestion.
An easier way for you would be to add a checkbox "Toolbar" beside "Thumbnail Panel" that shows/hides the toolbar (including the zoom slider and the element properties).
But this means I need two mouse clicks to hide the Toolbar and the Thumbnail Panel and again two mouse clicks to show them again.
A floating solution is in fact the better way (better usability).