Users can choose the image where they would like the slideshow to start and select the share icon (upper right) where you typically send the image to another person in iMessage. If you scroll down a bit you will see the slideshow option.
This is where you can then see the images from the day (or from the ones you selected). If you look in the lower right you will see options (ability to change these, music, speed, and ability to repeat).
Embedded below is a 54 second YouTube tutorial.