Hier ein kleines Snippet bzw. eine PHP Funktion um Thumbnails für Bilder zu erstellen, wahlweise mit Crop oder Resize Funktion :)

<?php
/**
 * Create Thumbnails
 *
 * @param string $filename 
 * @param string $filenameThumb 
 * @param int $thumbSizeWidth 
 * @param int $thumbSizeHeight 
 * @param bool $crop 
 * @return bool 
 */
function mkthumb($filename, $filenameThumb, $thumbSizeWidth = 100, $thumbSizeHeight = 100, $crop = false) 
{
	//Set Filenames / Folders
	$srcFile = $filename;
	$thumbFile = $filenameThumb;
	
	//Determine the File Type
	$type = substr($srcFile, strrpos($srcFile, '.') + 1);
	
	//Create the Source Image
	switch ($type) 
	{
		case 'jpg':
			$src = imagecreatefromjpeg($srcFile);
			break;

		case 'jpeg':
			$src = imagecreatefromjpeg($srcFile);
			break;

		case 'png':
			$src = imagecreatefrompng($srcFile);
			break;

		case 'gif':
			$src = imagecreatefromgif($srcFile);
			break;
	}

	//Determine the Image Dimensions
	$oldWidth = imagesx($src);
	$oldHeight = imagesy($src);

	//Crop: Calculate the New Image Dimensions
	if ($crop)
	{
		$original_aspect = $oldWidth / $oldHeight;
		$thumb_aspect = $thumbSizeWidth / $thumbSizeHeight;

		 //If image is wider than thumbnail (in aspect ratio sense)
		if ($original_aspect >= $thumb_aspect)
		{
		   $newHeight = $thumbSizeHeight;
		   $newWidth = $oldWidth / ($oldHeight / $thumbSizeHeight);
		}
		//If the thumbnail is wider than the image
		else
		{
		   $newWidth = $thumbSizeWidth;
		   $newHeight = $oldHeight / ($oldWidth / $thumbSizeWidth);
		}

		//Create the New Image
		$new = imagecreatetruecolor($thumbSizeWidth, $thumbSizeHeight);

		//Transcribe the Source Image into the New (Square) Image
		imagecopyresampled($new, $src, 0 - ($newWidth - $thumbSizeWidth) / 2, 0 - ($newHeight - $thumbSizeHeight) / 2, 0, 0, $newWidth, $newHeight, $oldWidth, $oldHeight);
	}
	//Resize: Calculate the New Image Dimensions
	else
	{
		//If image is wider than thumbnail (in aspect ratio sense)
		if ($oldWidth >= $oldHeight)
		{
			$newWidth = $thumbSizeWidth;
			$newHeight = $oldHeight * $thumbSizeWidth / $oldWidth;
		}
		//If the thumbnail is wider than the image
		else
		{
			$newHeight = $thumbSizeHeight;
			$newWidth = $oldWidth * $thumbSizeHeight / $oldHeight;
		}

		//Create the New Image
		$new = imagecreatetruecolor($newWidth, $newHeight);

		//Transcribe the Source Image into the New Image
		imagecopyresampled($new, $src, 0, 0, 0, 0, $newWidth, $newHeight, $oldWidth, $oldHeight);
	}

	switch ($type) 
	{
		case 'jpg':
			$src = imagejpeg($new, $thumbFile, 80);
			break;

		case 'jpeg':
			$src = imagejpeg($new, $thumbFile, 80);
			break;

		case 'png':
			$src = imagepng($new, $thumbFile, 80);
			break;

		case 'gif':
			$src = imagegif($new, $thumbFile, 80);
			break;
	}

	imagedestroy($new);

	return $src;
}

//Example Resize
mkthumb('test.jpg', 'test_thumb.jpg', 1024, 1024, false);

//Example Crop
mkthumb('test.jpg', 'test_thumb.jpg', 200, 150, true);
?>