Force image link download
You can use DownloadFile module. This module provides display formatter, for images, file etc, that uses downloadable links(of type: /download/file/fid/[fid]) and should be compatible with older browsers.

Note: 


If you are doing custom coding, you can use file id (fid) to create your download link.
You must upload the image, so that it can be assigned a file id.