bigger image in small screen

On windows size bigger than 768px my images is on column and need to have this size: 270px x 140px.
but in windows size less than 768px my image is full width

So i have set 2 size in my function.php

add_image_size( ‘article-image’, 270, 140, true );
add_image_size( ‘article-image-big’, 768, 398, true );

How can i load the “article-image-big” for window size less than 768px and “article-image” for windows size more than 768px ?

https://www.ads-software.com/plugins/ricg-responsive-images/