In a multipage website, while a person is viewing one page, or perhaps reading it, that page can be secretly downloading into the local browser "cache" memory, the graphics for any pages in the website, so that when the person goes to those pages, they don't have to wait for the pictures to slo---w--ly download. They just pop right up.
At the bottom of the first page, you put a conventional statement for placing an image on that page, except that you specify an image display size of one pixel by one pixel. Then it downloads, but never shows up on that page. It will be ready to rock when it's called on by a later page. Put this statement at the bottom of the page, so the page will be fully loaded in and functional for the viewer, before the "preloading" graphic download starts happening in the background.
The statement will look like this:
<img src="images_folder/particular_image.jpg" width="1" height="1">
An example can be seen on the page decribing how to create a slide show. There are three slides in that example. You can see the three pictures at the bottom of that "how to" page, each picture one pixel by one pixel big. The pictures are fully loaded into the browser cache memory, but are being displayed that small. To see the actual HTML statements click on Menubar: View>source.