You can import single or multiple images by clicking on the Import button and choosing Import Images:
You can select more than one image in the normal way (i.e. holding down CMD whilst selecting images). Each image will be placed in a separate section, with new sections being created if necessary. Multiple images will be imported in alphabetical order.
Moving Multiple Images
If you click near the top of a slide, that slide, and all the slides below it, will be darkened. This indicates that you can then drag them to a new position, either down or up. If up, you will overwrite any slides already there. The keyboard alternative to dragging with the mouse is to select a slide, then use CMD Up Arrow and CMD Down Arrow to move that slide, and all the slides below it, up and down.
Cut, Copy and Paste
You can use the usual Cut, Copy and Paste to move images around. These are available on the context (right-click) menu or with the usual shortcuts, CMD+x (cut), CMD+c (copy) and CMD+v (paste).
Drag and Drop
You can drag and drop images. If you drop onto an empty section then the image will be moved. If you drop onto a section that already has an image then the two images will swap positions.