Basically, what this script does, is to look if there is any ID named "image-gallery" and get the different list items that may exist within it. These elements will be displayed as thumbnails and a function will decide what to do when they are clicked. So, each time we click on a thumbnail, the "active" class will be assigned and the thumbnail should be visible in the larger container.
Step 7. Finally, save the changes by clicking the "Save template" button.
And here's the HTML code providing a normal list with the image-gallery ID, enclosed within a DIV with a relative position, so that it will avoid side effects of other pre-existing positions.
Step 8. Paste the below HTML to where you want to display the gallery by going either to Layout and adding a new gadget (click on the Add a gadget link and choose HTML/JavaScript), or inside a post/page in the HTML section.