Here is an example of how to use imagecopyresampled (). If you are having trouble understanding, be sure to pay attention to the comments in the code, they explain everything that is going on in each step.
<?php
// The file you are resizing
$file = 'yourfile.jpg';
//This will set our output to 45% of the original size
$size = 0.45;
// This sets it to a .jpg, but you can change this to png or gif
header('Content-type: image/jpeg');
// Setting the resize parameters
list($width, $height) = getimagesize($file);
$modwidth = $width * $size;
$modheight = $height * $size;
// Resizing the Image
$tn = imagecreatetruecolor($modwidth, $modheight);
$image = imagecreatefromjpeg($file);
imagecopyresampled($tn, $image, 0, 0, 0, 0, $modwidth, $modheight, $width, $height);
// Outputting a .jpg, you can make this gif or png if you want
//notice we set the quality (third value) to 100
imagejpeg($tn, null, 100);
?>
