Need help with thumbnail creation - pulling an all-nighter

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I'm having trouble with the functions for resizing a picture uploaded to my
server. Consider the following:

 $jpeg = imagecreatefromjpeg($filePath);  //I know this works!
 $oldX = imageSX($jpeg);
 $oldY = imageSY($jpeg);
 $thumbH = 100;
 $thumbW = ($oldX / $oldY) * 100;  //I know the dimensions are correct

 $thumbnail = ImageCreateTrueColor($thumbW, $thumbH);
 if(!ImageCopyResampled($thumbnail, $jpeg, 0, 0, 0, 0, $thumbW, $thumbH,
  $oldX, $oldY)) {
     print 'Error message<br />';
 //No error is produced, so the system thinks the image copy worked.

 imagejpeg($thumbnail, 'thumb.jpg');

I get an thumbnail image that is completely black. I did not get an error
from the copy, so the system thinks it works - but nothing was copied.

Can someone please give some help?

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