To start a scan click on the 
icon
Once a scan has been read in the program presents you with the scan and any of the photos within that scan that it has been able to extract automatically (to do this follow these rules). You can see the four automatically extracted images in the screenshot below.
To save any of the extracted images right click on one of them (on the right of the screen) and you will get the sub image dialog.
To change the area extracted click on one of them, a selection rectangle will appear in the main scan image:
Here is a closer look at the selection rectangle:
To remove one of the sub images highlight it by clicking on it (its frame willl turn red), then hit the DEL key.
Note that you do not have to limit yourself to saving only 4 images from a scan of 4 photos. You can also save details from within the images, as shown in this screenshot:
Although the original scan is only 4 photographs you can select and save different parts of them. In the above example I have selected 8 images from 4 photographs.