Yes, the addon is necessary to retrieve the image name to be preloaded.
So install the addon and use whatever pre-load script you wish.
My example pre-loads the next ("previous") image as the page loads so essentially, you are loading 2 photos instead of 1. This will slow down your page loading times for visitors with slow connections. It should not bother high-speed users though.
What Jay is showing you is an alternative method that can be used to load your images in the background after the page has loaded.
This is nice but the code he posted above only works with some mozzila based browsers as far as i know.