If you google "image gallery", possibly together with jQuery (if you want), you will certainly find hundreds of different examples.
Otherwise there is no easy answer unless you add some more information
- Will the thumbnails be separate pictures or do you just want the browser to downscale?
- Will the images (image paths) be stored in a file system or a database of some kind?
- Do you want all the images (on all sub pages) to be loaded on page load or do you want to load the images only when they are "about to be shown"?
- Do you already have the markup for the gallery (i.e. the container elements and the positioning CSS)?
I am asking this because usually you will get help if (and only if) you show some effort, i.e. some of your own code, and then describe your specific problem. We are not a free coding service.