createLandscape

kirby()->hook('panel.file.upload', function($file) {
    try {
        list($width, $height) = getimagesize($file->width());
        if ($width > $height) {
            // Landscape
            $file->resize(1200, 800);
        } else {
            // Portrait or Square
            createLandscape($file->url());
        }
    } catch (Exception $e) {
        throw new Exception($e->getMessage());
    }
});

function createLandscape($filePath){
    //$filePath = '003.jpg';  //full path to your png, including filename and extension
    $fileParts = pathinfo($filePath);
    $savePath = $filePath;  //full path to saved png, including filename and extension
    $colorRgb = ['red' => 229, 'green' => 231, 'blue' => 232];  //background color

    if($fileParts['extension'] == 'jpg' || $fileParts['extension'] == 'jpeg'){
        $img = @imagecreatefromjpeg($filePath);
        $imgWidth = imagesx($img);
        $imgHeight = imagesy($img);
        $width  = 1200;
        $height = 810;
        $endPosX = ($width - $imgWidth) / 2;
    }

    //create new image and fill with background color
    $backgroundImg = @imagecreatetruecolor($width, $height);

    $color = imagecolorallocate($backgroundImg, $colorRgb['red'], $colorRgb['green'], $colorRgb['blue']);
    imagefill($backgroundImg, 0, 0, $color);

    //copy original image to background
    imagecopy($backgroundImg, $img, $endPosX, 0, 0, 0, $imgWidth, $imgHeight);

    //save as jpg
    imagejpeg($backgroundImg, $savePath, 100);
}