Click on the title sections (thumbnails) to start with, the slideshow will gradually display all photos in that section (album), you will get back to the starting screen at the end of the album,
In general: follow all "tool tips" at your mouse cursor,
To get back to the title sections from the main screen click on the "left bottom" icon (tip: gallery).
Choose another page of gallery sections (albums) by clicking on "next screen" or "previous screen" buttons,
The little thumbnails will allow you to choose specific photo, but then you will need to click on main screen or on "play" (right button) as at this point the slidshow will pause...
FULL SCREEN mode is enabled by clicking on right button, choose "esc" on your keyboard to exit full screen... this will render full screen pictures, but at slower rate.