You can hide the menu bar (more precisely: activate the auto-hide mode) by checking the "Auto-hide the menu bar" option in the program preferences, see Menu->Options->Miscellaneous.
Alternatively you can right-click on the menu bar itself and click "Auto-hide Menu Bar" on the popup menu.
The menu bar will auto-hide itself when not used. Just briefely press the Alt key to pop it up, or move the mouse cursor towards the bottom of the window. When used in conjuction with the full-screen kiosk mode, you get a true 100% full screen view of your images.