It would be good to know if your website is made in wordpress , prestaghop or custom. Or in any other way.
If it's wordpress, you could create images of the size that you use on the web, you could add in your functions.php file
/We record the size
function bp_body_size(){
add_image_size('body-size', 700);}
add_action( 'after_setup_theme', 'bp_body_size' );
//Add size to image options
function bp_body_size_choose( $sizes ) {
return array_merge( $sizes, array(
'body-size' => 'New body size');}
add_filter( 'image_size_names_choose', 'bp_body_size_choose' );
With this you would have a new image size on your website of 700px x XXXpx.You would have to regenerate all the images of the web
Greetings