Using XnView
From XnView Wiki
Display Modes
XnView presents two different user interfaces, or windows, depending on context: Browser Mode and Viewer Mode. When XnView is started, it will normally begin in Browser mode and show the contents of a folder.
XnView's menu bars and tool bars are context-sensitive, and change depending on the mode The toolbars are also dependent on context. Grayed-out menu items are not available in the current mode or context.
Most of XnView's common functions and menu/toolbar items are available using Shortcut Keys.
Tool tips are available throughout XnView. To display the function of a toolbar icon, simply move the mouse over the button and leave it there for a moment.
A third display option, Basic Mode, is rarely need or used, but is mentioned here for purposes of being thorough.
Viewer Mode
Image Viewer mode is invoked when an image is opened. You can open an image by either double-clicking on it from Browser mode, or by using the standard File > Open menu.
Information about the image is displayed in the status bar at the bottom of the window. For more information, see Image Information.
XnView's shortcut keys are context-sensitive. For the shortcut keys that work in Image Viewer mode, see Viewer Mode Shortcut Keys and All Modes Shortcut Keys.
Basic Mode
Basic mode is used when the Browser and Viewer modes are not active. The only way to see this mode is by closing all the open windows—the browser plus as many images as may be open; what you have left is an empty window with a few buttons. This is the "basic" mode, and looks the image below. See All Modes Shortcut Keys for the shortcut keys that work in Basic mode.