Each image could be selected/deselected with left mouse button (if configured to act with this behaviour). Startup state depends from INI file.
With Select all button you can select all the images, with Clear Selection button you can deselect all the images.
The border of unselected images is blue, the border of selected images is yellow.
With the Show selected button and Show all button you can choose if show only selected images or all the images.
Some operations are applicable only on selected images.