External URL

Hi wasanajones,

Not out of the box, but you could code it in.
If you put this into your functions.php (Or in a lose plugin file.)
And change the urls.

function dfi_external_image($html, $post_id, $default_thumbnail_id, $size, $attr) {
    // add a class to the existing class list

    $alternate_url           = 'https://i.imgur.com/mtCQTvp.jpg';

    // add your own other sizes here (use <code>get_intermediate_image_sizes()</code> to check the defined sizes)
    if ($size == 'thumbnail') {
        // set smaller image
        $alternate_url = 'https://i.imgur.com/mtCQTvps.jpg';
    }
    
    $new_html = '<img src="' . $alternate_url . '" ';
    // @todo add correct image size attributes
    foreach ( $attr as $att => $value ) {
        $new_html .= "$att='$value' ";
    }
    $new_html .= '/>';

    return $new_html;

}
add_filter( 'dfi_thumbnail_html', 'dfi_external_image', 10, 5 );

This will work out of the box, but it is a bit dirty, needs more sizes set, and should have the width and height attributes.