You can sort pictures by clicking on the picture and dragging to the file tree on the right.
You can also select a group of pictures from the file list on the left and drage the group to the file tree on the right.
You can change the directory shown in the file list by double-clicking on a directory in the file tree. You can also right click on the directory you want and select “Show Single Directory” from the pop-up.
The Slide Sorter allows you to sort a group of pictures into a particular order and then rename the group of pictures so that they will be displayed by the file system in order.
To turn the sorter on or off select the View menu and check or uncheck the Slide Sorter box (circled in red below).
As you drag pictures to the sorter, the pictures are marked with a green check mark. You can drag pictured around in the slide sorter. Here is what the buttons do: