The problem with a thumbnail option, particularly where many images are involved, is that it is difficult to create the thumbnail gallery at a size that can be used effectively on mobile screens. Essentially, a block of usable and easily isolated thumbs under a main image often means users having to scroll up and down between the thumbnail grid and the larger image. If the images are too small to be hit positively with fat fingers, the whole thing becomes a bit of a pointless exercise. In the example provided, the site isn’t responsive so the issue doesn’t arise, but if you’re making a site for cross device use, it may be better to rethink the available options.
We were faced with a similar request from a client recently and we finally settled on a workable compromise that works at all screen sizes. Essentially, we created a horizontal scrolling thumbnail strip that responds to all screen sizes whilst still making the thumbnails easily selectable. The larger images were then displayed in a lightbox where they could be scrolled at full size, or the box could be closed, allowing the user to go back to the thumbnail strip for quick selection of another image. The good thing about this solution is that the images don’t become so small on mobile screens as to be difficult to see or select.
I posted the gallery here some time ago, but if you missed it and would like to give it a go, you can download it from HERE. You can download the standard 20 image gallery, or, if you have Temult Hype, you can also download the Hype project file to customise it to accommodate as many images as you wish.