We can use add_image_size() function to define additional image sizes along with the default sizes: thumbnail, medium, large and full size. Use the image_size_names_choose filter to access the array which contains the default image sizes, and add the additional image sizes to it.

Add this code to your theme’s functions.php file.

if ( function_exists( 'add_image_size' ) ) {
    add_image_size( 'new-size', 500, 300, true ); // Set custom image size

function my_image_sizes($sizes) {
        $addsizes = array(
                "new-size" => __( "New Size")
        $newsizes = array_merge($sizes, $addsizes);
        return $newsizes;
add_filter('image_size_names_choose', 'my_image_sizes');

Filter image_size_names_choose is in the image_size_input_fields() function in wp-admin/includes/media.php and adding additional or custom image sizes to the media uploader is now very easy.


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