我设法找到了问题的解决方案，尽管我最终使用了PHP GD函数而不是wordpress函数。我希望这能帮助其他试图做类似事情的人。

$src    = // Source image URL
$max_w  = // Output maximum width
$max_h  = // Output maximum height
$dir    = // Output directory
$fle    = // Output filename

function createThumbnail( $src, $max_w, $max_h, $dir, $fle ) {
    $img_url = file_get_contents( $src );
    $img = imagecreatefromstring( $img_url );
    $old_x = imagesx( $img );   // Source image width
    $old_y = imagesy( $img );   // Source image height

    // Conditions for maintaining output aspect ratio
    switch ( true ) {
        case ( $old_x > $old_y ):   // If source image is in landscape orientation
            $thumb_w = $max_w;
            $thumb_h = $old_y / $old_x * $max_w;
            break;
        case ( $old_x < $old_y ):   // If source image is in portrait orientation
            $thumb_w  = $old_x / $old_y * $max_h;
            $thumb_h  = $max_h;
            break;
        case ( $old_x == $old_y ):  // If source image is a square
            $thumb_w = $max_w;
            $thumb_h = $max_h;
            break;
    }

    $thumb = imagecreatetruecolor( $thumb_w, $thumb_h );

/**
    I quoted this one out since I ran in compatibility issues, I\'m using PHP 5.3.x
    and imagesetinterpolation() doesn\'t exist in this version

    imagesetinterpolation( $thumb, IMG_SINC ); // Recommended Downsizing Algorithm
**/

    imagecopyresampled( $thumb, $img, 0, 0, 0, 0, $thumb_w, $thumb_h, $old_x, $old_y );

    $result = imagejpeg( $thumb, $dir . $fle );

    imagedestroy( $thumb ); 
    imagedestroy( $img );

    return $result;
}