When are images downloaded?

[Code.Warrior]

I'm thinking of using javascript to display a larger popup picture when the mouse hovers over a thumbnail version. Problem is there will be quite a few thumbnails on the page and downloading all the larger images will take a long time.

If the img src is in a javascript function will this only retrieve the image when the mouseover triggers the function call?

[funkdaddy]

It's loaded whenever you set the img.src, either an element or an image object. So if you have a function that sets the source, then it will only load when the function is called.

[Code.Warrior]

So it will only request the image from the server when the function is called?

Is that in all cases and is there a way to test this as Firefox seems to indicate the total page size includes those images which might not be called.