The image at the top of the Memories tab is a teaser for a themed compilation, "On This Day" slideshow, or 1s Movie. Tapping that image will not change the screen.
If there are multiple images, the displayed image will change automatically, but it cannot be interacted with.
If you want to view one of the Memories from the top, scroll down and tap the thumbnail for that Memory.
Some Memories appearing at the top may not be viewable for free. Please look for a lock icon or other indicator to tell if a Memory can't be viewed.