Image Resize and save by using GD function and Percentage based

<?php
$filename = 'test.jpg';
$percent = .5;
list($width, $height) = getimagesize($filename);
$newwidth = $width * $percent;
$newheight = $height * $percent;
$thumb = imagecreatetruecolor($newwidth, $newheight);
$source = imagecreatefromjpeg($filename);
imagecopyresized($thumb, $source, 0, 0, 0, 0, $newwidth, $newheight, $width, $height);
$ourFileName = "newtest.jpg";
imagejpeg($thumb, $ourFileName);
imagedestroy($thumb);
?>

PNG to Jpeg in PHP using GD library

<?php
function png2jpg($originalFile, $outputFile, $quality) {
    $image = imagecreatefrompng($originalFile);
    imagejpeg($image, $outputFile, $quality);
    imagedestroy($image);
}
png2jpg("img/test.png", "img/".time().".jpg", 100);
if ($handle = opendir('img/')) {
	$i=0;
   while (false !== ($entry = readdir($handle))) {
      if ($entry != "." && $entry != "..") {
         echo "<img src='img/$entry' width=150 height=100>    ";
			if($i%10==0 && $i!=0){
				echo "<br>";
			}
			$i++;
      }
   }
   closedir($handle);
}
?>

To add watermark image with original image by using php GD functions

$main_img = "Porsche_911_996_Carrera_4S.jpg"; // main big photo / picture
$watermark_img = "watermark.gif"; // use GIF or PNG, JPEG has no tranparency support
$padding = 80; // distance to border in pixels for watermark image
$opacity = 20; // image opacity for transparent watermark
$watermark = imagecreatefromgif($watermark_img); // create watermark
$image = imagecreatefromjpeg($main_img); // create main graphic
if(!$image || !$watermark) die("Error: main image or watermark could not be loaded!");
$watermark_size = getimagesize($watermark_img);
$watermark_width = $watermark_size[0];
$watermark_height = $watermark_size[1];
$image_size = getimagesize($main_img);
$dest_x = $image_size[0] - $watermark_width - $padding+10;
$dest_y = $image_size[1] - $watermark_height - $padding-80;
// copy watermark on main image
imagecopymerge
($image, $watermark,$dest_x, $dest_y, 0, 0, 
$watermark_width, $watermark_height, $opacity);
// print image to screen
header("content-type: image/jpeg");
imagejpeg($image);
imagedestroy($image);
imagedestroy($watermark); 

To Crop image using PHP GD function

cropImage(150,100, 'example.png', 'png', 'image.png');
function cropImage($nw, $nh, $source, $stype, $dest) {
$size = getimagesize($source);
echo "
".$w = 500;
echo "
".$h = 400;
switch($stype) {
case 'gif':
$simg = imagecreatefromgif($source);
break;
case 'jpg':
$simg = imagecreatefromjpeg($source);
break;
case 'png':
$simg = imagecreatefrompng($source);
break;
}
$dimg = imagecreatetruecolor($nw, $nh);
echo "
".$wm = $w/$nw;
echo "
".$hm = $h/$nh;
echo "
".$h_height = $nh/2;
echo "
".$w_height = $nw/2;
if($w> $h) {
echo "
".$adjusted_width = $w / $hm;
echo "
".$half_width = ($adjusted_width / 2)+25;
echo "
".$int_width = $half_width - $w_height;
imagecopyresampled($dimg,$simg,-$int_width,0,0,0,$adjusted_width,$nh,$w,$h);
}
elseif(($w <$h) || ($w == $h)) {
$adjusted_height = $h / $wm;
$half_height = $adjusted_height / 2;
$int_height = $half_height - $h_height;
imagecopyresampled($dimg,$simg,0,-$int_height,0,0,$nw,$adjusted_height,$w,$h);
} else {
imagecopyresampled($dimg,$simg,0,0,0,0,$nw,$nh,$w,$h);
}

imagejpeg($dimg,$dest,100);
}