In WordPress 4.4 images are displayed with the srcset attribute. This attribute allows the browser to select alternative images for best viewing. Sometimes though this may not be wanted as the image being displayed is the only image you want to show for all users.
To not have any srcset images, use the following:
add_filter( ‘wp_calculate_image_srcset_meta’, ‘__return_null’ );
For other filters new to WordPress 4.4, you can visit the blog post:
One user had a problem with the images in the srcset not having the https protocol and causing security warnings. This stack overflow post resolves that: