I have php code which create pdf thumbnail as follows;
setImageFormat(\"png
In my case I found out a solution like this:
$im = new Imagick("http://www.yourserver.com/upload/file_name.pdf");
$im->setResolution(300, 300); // if higher image will be good to read
$im->setIteratorIndex(0); // read first page
$im->setImageFormat('jpg');
header('Content-Type: image/jpeg');
ob_start();
print $im->getImageBlob();
$contents = ob_get_contents();
ob_end_clean();
echo "
"; //output as image
good luck.