When are slideshow images sent to the client?

I have a slideshow that currently rotates through 10 images, using a web service (.asmx) to provide the images.  The GetSlides method returns an array of AjaxControlToolkit.slide.

Does this return all of the actual images (.jpg and .bmp) when the page opens, or are the actual images fetched as needed?

Why I ask: if I put a large number of images in the source folder, will this slow the page unacceptably if they are all sent at once.

Thank you for your attention, Robert.