I had a problem when using $im->setImageFormat('jpeg');
Image colors got inverted when i converted pdfs to jpg thumbs.
Had to add $im->setImageColorspace(255); to solve it.
<?php
// read page 1
$im = new imagick( 'test.pdf[ 0]' );
// convert to jpg
$im->setImageColorspace(255);
$im->setCompression(Imagick::COMPRESSION_JPEG);
$im->setCompressionQuality(60);
$im->setImageFormat('jpeg');
//resize
$im->resizeImage(290, 375, imagick::FILTER_LANCZOS, 1);
//write image on server
$im->writeImage('thumb.jpg');
$im->clear();
$im->destroy();
?>