How to handle different image types?

[Anon]

Hi!
I get from DB image files PNG or JPG. By name i can't differentiate it. Code like this doesn't work:
$im = ImageCreateFromJpeg($picname);
$text= "Some text";
if($im)
{
$nsi= ImageColorAllocate($im,255,190,0);
ImageString($im,3,0,0,$text,$nsi);
Header("Content-type: image/jpeg");
ImageJpeg($im);
ImageDestroy($im);
}
else
{
$im = ImageCreateFromPNG($picname);
$nsi= ImageColorAllocate($im,255,190,0);
if(!isset($text))
$text="NSI Ltd.";
ImageString($im,3,0,0,$text,$nsi);
Header("Content-type: image/png");
ImagePng($im);
ImageDestroy($im);
}