A neat little thing found at W3schools. A click on a thumbnail image will open the image in larger. More images can be added. For the thumbnail images it will look the best if you let the width of them all add up to 100%.
It is possible to place text, or anything else, between the containers .thumbs-holder and .image-holder. The sequence can also be reversed.
The vertical version of this gallery has the thumbnails in a strip down the left side of the main window. It is easy to pop in more thumbnails.