When the lower pane is showing imported thumbnails, you can select a single thumbnail by single-clicking it.  If you hold the SHIFT key while clicking, you extend the selection so that a set of adjacent images is selected. If you hold the CTRL key while clicking you toggle the selection of a single image, changing it from unselected to selected or vice versa. You can drag a set of selected images to change their order in the imported thumbnails pane.