Galleries may appear as a dedicated gallery navigated to from the Galleries Page.
They may also appear in report pages. In any case:-

Click on any image for a larger version.
Scroll through these by using the chevron that appears when your cursor is at either side of the image.
You may also close the large image and click on a different one.

OR click [Show slideshow] This moves at a fairly sedate rate but you my click on the image to move to the next one immediately.