Use Picture Tag and responsive Images in Gallery

It would be nice if the Gallery would use the picture Tag and the sizes defined in the functions.php.

Like if you have your responsive images set up like described here [https://www.elegantthemes.com/blog/tips-tricks/integrate-picturefill-with-wordpress-and-make-images-responsive](https://www.elegantthemes.com/blog/tips-tricks/integrate-picturefill-with-wordpress-and-make-images-responsive), the gallery could hook into the add_picture_sources function and we would have nice responsive images for every viewport.

Start by reading functions.php and locating the Gallery implementation and its add_picture_sources hook. Confirm how the sizes defined there are exposed to gallery images, then verify that the gallery outputs picture elements with responsive sources for each viewport described in the issue.

Written by the indexing model from the issue text.