While downloading images from internet, it's easiest to let php decide what is the file type. So, forget using imagecreatefromjpg, imagecreatefromgif and imagecreatefrompng. Instead, this is the way to go:
<?php
$src = "http://www.varuste.net/tiedostot/l_ylabanneri.jpg";
$image = imagecreatefromstring(file_get_contents($src));
?>
