How do I allow a user to download my images, not open it in a browser?
Browsers are designed to display the images when they can. You'll have to instruct your visitors to save the file themselves, or simply leave it to their option, as they may want to see it before committing disk space to its storage. They can always save it after viewing it normally.
Tell your viewers to
Right click > Save Picture As. Works in IE, other browsers have similar schemes.
If anybody tells you to zip the file, ignore them. How do you know your viewer has the required unzip software installed?. Most Mac users and some PC users don't have unzipping software, and there's little benefit compressing an already compressed image.