In my solution, I use two scripts. One for showing the image true size and another for generating a thumbnail -- I may be wrong, but I think it's better to generate a thumbnail as needed on the fly than it is to store both images (large and thumbnail) in the dB.
Cache it on the filesystem even if it's for a short time (of course if the image is updated elsewhere the cache needs to be cleared as well..).
<?php
$file = '/path/to/cache/file.jpg';
if (file_exists($file)) {
if (filemtime($file) > strtotime('-30 minutes')) {
$fp = fopen($file, 'rb');
fpassthru($fp);
exit;
}
// the file is older than 30 minutes, kill it and start again.
unlink($file);
}
// continue creating your thumbnail.
$fp = fopen('/path/to/cache/' . $filename, 'wb');
fputs($fp, $imagecontents);
fclose($fp);
// display image
--
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