Put off loading my theme banners to speed up Drupal
I think you're looking for a technique called "lazy-loading" to implement on your pages. Lazy loading means that the images are only loaded upon viewing them. 

You can use the Lazyloader module as a start to implement it, however how it will exactly work for your site is completely dependent on your Drupal theme and module(s) configuration. One useful example (and starting point) is this blogpost, which explains how to implement the layloader module in combination with a slideshow.