rebuilding boot.img fails

[Guest smokku]

Using instructions at http://android-dls.com/wiki/index.php?titl...ack_Boot_Images

I've rebuild boot.img for my Dell Streak, but the size of the new image is different than the original:

-rw-r--r-- 1 smoku smoku 2545664 2008-02-29 02:33 boot.img

-rw-r--r-- 1 smoku smoku 2547712 2010-08-28 12:55 mynewimage.img

This reconstructed image fails to boot. :/

Does anyone know what is wrong?