Browsers recognize jpg URL's and hand them over just like a .htm, so I don't think you can force it on the user-end (not positive on that, though). Maybe you could do it with a server-side script somehow?
I'm a little confused by your post, do you want a download prompt to appear and the image to be downloaded to a location on the user's hard drive, or you you want the image to open or be displayed in a pop-up window?
If the former, you can attempt to force the download of files if the server is running Apache: see this recent thread in the Apache server forum:
The problem is that browsers will "sniff out" the file, even if you change the extension, and try to manage it accordingly. To effectively force the download of a known MIME type, you need to deliver it via server side programming and "munge" the content-type header. This makes the browser force the download dialogue.