Resizes a picture to fit in a box, clipping the edges if necessary. When picture smaller than the given box, centers the picture and adds a transparent background.

boxed_thumbnail.php

<?php function boxed_thumbnail($data, $size)
{
	$src = imagecreatefromstring($data);
	$src_width = imagesx($src);
	$src_height = imagesy($src);
	
	// Resize algo:
	$src_ratio = $src_width / $src_height;
	$trg_ratio = $size[0]   / $size[1];
	
	// Fall back values:
	$dst_width = $src_width;
	$dst_height = $src_height;
	if ($src_ratio >= $trg_ratio)
	{
		if ($src_height > $size[1])
		{
			$dst_width = $size[1] * $src_ratio;
			$dst_height = $size[1];
		}
	}
	else
	{
		if ($src_width > $size[0])
		{
			$dst_width = $size[0];
			$dst_height = $size[0] / $src_ratio;
		}
	}
	
	$dst = imagecreatetruecolor($size[0], $size[1]);
	$bg_color = imagecolorallocatealpha($dst, 255, 255, 255, 127);
	imagefill($dst, 0, 0, $bg_color);
	
	imagecopyresampled(
		$dst,
		$src,
		($size[0] - $dst_width) / 2,     // dst_x
		($size[1] - $dst_height) / 2,    // dst_y
		0,                               // src_x
		0,                               // src_y
		$dst_width,
		$dst_height,
		$src_width,
		$src_height
	);
	
	ob_start();
	imagepng($dst);
	$data = ob_get_clean();
	
	imagedestroy($src);
	imagedestroy($dst);
	
	return $data;
}

boxed_thumbnail.py

from PIL import Image
from StringIO import StringIO

def boxed_thumbnail(data, size):
    src = Image.open(data)
    src_width = src.size[0]
    src_height = src.size[1]
    
    # Resize algo:
    src_ratio = float(src_width) / src_height
    trg_ratio = float(size[0]) / size[1]
    
    # Fall back values:
    dst_width = src_width;
    dst_height = src_height;
    
    if src_ratio >= trg_ratio:
        if src_height > size[1]:
            dst_width = int(size[1] * src_ratio)
            dst_height = size[1]
    else:
        if src_width > size[0]:
            dst_width = size[0];
            dst_height = int(size[0] / src_ratio)
    
    dst = Image.new('RGBA', size, (255, 255, 255, 0))
    dst.paste(
        src.resize((dst_width, dst_height)),
        (
            (size[0] - dst_width) / 2,
            (size[1] - dst_height) / 2,
        )
    )
    
    buf = StringIO()
    dst.save(buf, format='PNG')
    buf.seek(0)
    
    return buf