Hi,
Well, technically, ebay doesn't allow use of <head> tag, and they/Auctiva will/should delete the head code and script when it's placed there.
Just place the script after the 1st HTML tag in your template for protection from deletion. If you look at the old template, use that as an example.
The "getElementById" is essentially looking up the ID of the tag where your main image is in the document. If you look in the code at the IMG tag for the main image, it has id="bigPic". That's how that function knows which image is the target for that replacement. src="..." is the URL of the image(s). So, the replace is literally the string for the URL on the statement. You previously defined the array of images as pic[1], pic[2], etc. and stored the URL string unique to each, so the call from mouseover just passes the function the number (num variable) of the image.
Putting it all together....all the images links are preloaded and their source URLs are added to array pic (1 - 8). With mouseover of each thumb, you're calling function swap with a specific thumb/image number in the array asking the function to locate the main image and replace it's URL source/src string with the new image's.
Hope that makes some sense.
Danno