Figure 4.7 show's nip2's main image processing window. Drag with the middle mouse button to scroll the workspace window. Drop a file on to the workspace background (from your file manager) to load that file.
If you select one of these image processing operations, nip2 will apply that operation to the bottom few items in the current column (however many are necessary -- two items for Math / Arithmetic / Add, for example), or alternatively, if you have selected some objects explicitly, it will try to apply the operation to the selected objects. See §4.4.3. As you move the mouse pointer over menu items nip2 tries to display some helpful information about the operation, including the number and type of arguments the operation expects.
If you left-click on the label, it changes to display the space nip2 has free internally for performing calculations. You can change this limit in the Preferences workspace. Click again to switch back to disc free.
If you have objects selected, this area changes to show the names of the selected objects.