Show the "Save as..." dialog

Hi,

I have an image gallery featuring multiple thumbnails. Now, what I want to do is that when the user clicks one of the thumbnails, the “Save as…” dialog pops-up allowing the user to save the full size image. I have both the thumbnail and full size image on my server and can include the paths in an xml file. Is there a way to do it?

Any help is greatly appreciated! Many thanks in advance for your time and help!