The easy way would be to zip the image. The harder way involves either using PHP to force download, or to use Apache .htaccess file to force it, but these take some effort to setup. You can probably track down some tutorials though.
Make a link to an empty page with that code in the header
<?php
header('Content-type: application/jpg');
header('Content-Disposition: attachment; filename="your_image.jpg"');
readfile('your_image.jpg');
?>
make sure your image is on the server with the correct path