If you are using windows imagemagick, at leat in my case, you have to include the full path of the file, including the 'c:' at the beginning. If you don't know for sure the way that the filename is going to come, you can use realpath() function to normalize it:

<?php
    $incoming_file = '/Users/John/Desktop/file_loco.jpg';
    $img = new Imagick(realpath($incoming_file));
    // realpath() will convert '/Users/John/Desktop/file_loco.jpg' to 'C:\Users\John\Desktop\file_loco.jpg'
    // and ImageMagick will have no trouble when loading
?>

If you try and load an ICO file whose extension isn't .ico, you'll get an error going along the lines of no delegate existing for the supplied image's format.  The can occur, for example, if you're using a temporary file.

<?php
$tmp = tempnam('cache/images', 'ico_');

if (copy('http://remote.url/favicon.ico', $tmp)) {
    $ico = new Imagick($tmp);  // <-- ERROR!
}
?>

Your first thought might be to rename your temporary file's extension to .ico, but I decided to try something that works on the command line―prefix the file name with 'ico:'

<?php
$tmp = tempnam('cache/images', 'ico_');

if (copy('http://remote.url/favicon.ico', $tmp)) {
    $ico = new Imagick("ico:$tmp");  // <-- Works great!
}
?>

if you need to create an image from a string containing the image binary, use readImageBlob, eg

<?php
$imagick = new Imagick();
$imagick->readImageBlob($str);
?>

likewise if you need to create an image from a file handle (without having the filepath), use readImageFile()

you can use PDF page as image. To do that, install ghostscript libraries.
IMagick use ghostscript to read PDF page.

<?php
$myurl = '/pdf/mypdf.pdf';
$image = new Imagick(realpath($myurl).'[0]'); //[0] indicate the number of the wanted page
$image->setResolution( 300, 300 );
$image->setImageFormat( "png" );
$image->writeImage(realpath('./mypdf.png'));
?>