[Oblivion Knight]

Much love to KirbyDE for helping me out with my request..

This is the resulting plugin:

if ($vbulletin->options['allowsignatures'])
{
	$maxsigwidth = 400;
	$maxsigheight = 150;

	function check_sig_size($sig)
	{
		global $maxsigwidth, $maxsigheight, $errors, $vbulletin;
		$sig = str_replace(array('  ', '\\"'), array('', '"'), $sig);

		// check valid signature image size
		if ($imginfo = getimagesize($sig))
		{
			if ($imginfo[0] > $maxsigwidth or $imginfo[1] > $maxsigheight)
			{
				$vbulletin->GPC['preview'] = 'true';
				$errors[] = fetch_error('signaturetoobig', $maxsigwidth, $maxsigheight);
			}
		}
	}

	$url = preg_replace('#\[img\]\s*(https?://([^<>*"' . iif(!$vbulletin->options['allowdynimg'], '?&') . ']+|[a-z0-9/\\._\- !]+))\[/img\]#iUe', "check_sig_size('\\1')", $signature);
}

The phrase then becomes something similar to:

Your signature can not have images larger than {1}w x {2}h in pixels.