It’s easy to add SRCSET Support by replacing
$src = wp_get_attachment_image_src( $src, $size);
$src = $src[0];
with
$src1 = wp_get_attachment_image_src( $src, $size);
$srcset = wp_get_attachment_image_srcset( $src, $size);
$src1 = $src1[0];
in wpb_iw_shortcode.php.
The image can be loaded via
<img src="<?php echo $src1; ?>" srcset="<?php echo $srcset; ?>" sizes="50vw" alt="">
Maybe you could create an update to fully support SRCSET? There are still a view things (like automated sizes) missing in my code…
https://wordpress.org/plugins/wpb-image-widget/