CSS preload trick I learn from this post
It looks like old trick, since you can prefetching image.
But you can still try it :p
Just add a div(just one!) which class is "preload-img"
i.e.: <div class="preload-img"></div>
And it's done
Liquid will get all static asset which file name contain thumbnail and it's image or gif file, push them as preload-img's background, so browser will load them as soon as your site begin load, but it'll not show since their offset out of view.
Suggest put preload-img div somewhere not top of your site construct , since it potentially looks ugly.