You can add more than one folder with images. Each folder appears on the list with a checkmark at the left that allows temporarily disabling one folder without needing to remove it from the list.
You could select the entire hard disk to show all the images you have but then the program at startup would have to inspect all the folders and subfolders in your disk looking for images. This could take some time (depending on how many files are on your disk). Anyway once scanned the first time the program will keep a cached list of images so further sessions do not take this long time to start. But, if possible, it's a good idea to make a more specific selection of folders, those where you store the images.
The program allows setting some rules to filter the images of the folders selected avoiding some images to appear. Click on the first empty line under Filters and you will be able to set up these conditions.
First choose Must have or Discard: The former will instruct the program to use only the image files that fulfill the rule, the later would not include those files.
Next goes the rule. It may be Name containing: The filenames that contains the text you type in the next field. Name between: Useful when the file names of your photos are numbered, this would allow setting an interval of photographs, for example something like XXX-0500 and XXX-0750 would include or discard images whose filename is greater or equal to XXX-0500 and less or equal than XXX-0750
Note: If there are several "MustHave" rules a file needs to fulfill only one of them in order to pass (as far as it does not satisfy other possible "discard" rules)
There are two especial filters next: Discard images smaller than... to avoid using images too small that could exist in the folders selected.
Skip images in portrait orientation, if you feel the display is less attractive when these images appear you can set the program to discard them and use only images in landscape format (width larger than height)
A selection of folders can be saved in a file for later use either by loading it from this window or running that selection directly. To run the slideshow with a previously saved selection, locate the file where you saved the selection and double click it, this will run the slideshow loading this specific selection of files. This feature allows having different collections of images ready to run with a simple click of the mouse.