How to display download links for all sizes of an image?

I’m using a WordPress installation as a personal image database. Each image has a post, where the image is uploaded (full size) as a featured image that is then displayed in the post.

How would I go about creating “Click here to download” links for the various sizes of image being generated by WordPress? Is there a plugin that handles that that anyone would recommend, or can I just edit the single-page template to include this?