Use the scroll bars, mouse wheel, mouse clicks or arrow keys to move around and select the picture appearing in the top "Active Image" region.
Left-click clears shows the picture just clicked as the active image. Click the "Selection Box" in the upper left of each picture to select that picture for further processing, such as removing or hiding.
Ctrl-click on Windows, or command-click on a Mac, keeps all previous selections and adds the picture clicked.
Shift-click selects all pictures between the last selection and the new selection. Any selections not between the last selection and current selection are cleared.
Ctrl-shift-click on Windows, or command-shift-click on a Mac, keeps all previously selected pictures and adds all pictures between the last selection and the new selection.
The Zoom slider adjusts the size of the thumbnails in the overview.