The example uses a button procedure with the BS_BITMAP style that takes 2 identical sized images, in this case PNG images, to form the up and down appearance of the button. This technique allows you to escape from the default Windows button appearance. You must do your own artwork but it means you can produce any appearance you want.
The example uses images that are very conservative that suit apps that you use very often, you can make dazzling up and down images but you can get tired of them very quickly. The example also uses a single JPG image as the main window background with the button super-imposed over it.
To try and keep the clutter down, a number of procedures have been put into SLL modules and compiled into a library, the source for the SLL modules is present in the lib directory.