In parts of the Blender interface where pictures can be loaded, you generally have the option of using a FileSelect window or the ImageSelect window.
For the most part, the functionality is the same.
The ImageSelect window reads the directory and examines each file to see if it is a recognisable image. After the entire directory is scanned, the images are loaded and displayed as a thumbnail and saved in the ".Bpib" file.
If a ".Bpib" file already exists, it is first read in and compared to the contents of the directory.