While working with the Responsive Images Module in Core of 8.3.x, i recognized that the picture Element is actually not rendering correctly.
If the Fallback Image is configured (with an image style) within a specific Responsive Image Style, the src attribute for img within the picture element is filled with the path of the fallback image (always) instead of the correct data.
With configured Fallback Image using an image Style - this is NOT working:
<img src="/sites/default/files/styles/conductor_header_image_350_x_171/public/2017-10/image_example_0.jpg?itok=3WI0Xk6R" alt="Example Image" typeof="foaf:Image" class="img-responsive">
Without configured Fallback Image (set to -empty image- !) - this is working:
<img src="data:image/gif;base64,R0lGODlhAQABAIABAP///wAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==" alt="Example Image" typeof="foaf:Image" class="img-responsive">