File Force | drupal.org

This was surprisingly hard to find!

drupal open image in download link
drupal "open image" in "download link" headers
download image file browser Drupal
force file for download not image display

Stefan found it: file force module
http://drupal.org/project/file_force

When a site visitor requests a file, File Force can force the user's web browser to download the file to disk instead of displaying it in the browser window.