By default a 1px Data URI image is the placeholder for lazyloaded (Responsive) image. Useful to perform a lot better. Uncheck to disable, and use Drupal-managed smallest/fallback image style instead. Be sure to add proper dimensions or at least min-height/min-width via CSS accordingly to avoid layout reflow, or choose an Aspect ratio via Blazy formatters. <br>Since <b>2.10</b>, disabling this will no longer result in downloading fallback image (double downloads). Thus, allows you to have non <code>empty image</code> for fallback at Responsive image style UI without extra HTTP requests, while using <code>empty image</code> (enforced now) for the SRC. Basically marrying those options. not negating each other anymore.