Select media from your computer using the Import-Media button.
All imported media remains in your browser. This site has no backend server to receive data, it is entirely one page made with HTML, CSS, and JS.
The "full path to video directory" field is only applicable when this HTML page is run locally as a file://. By doing so, you can stream large files, and they aren't loaded into a browsers memory.
The first image you select after your import is your backdrop.
The second image you select is your sprite to control.
This site is not mobile friendly, and is not intended to be.