When the lower pane is showing computed thumbnails, you can select a single computed 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 (Windows) or COMMAND key (Mac)  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 computed thumbnails pane.