Prob Changing Header

Hello.

I am completely flummoxed as to changing my header.
I’ve done this before and it’s worked all right, but something is going very wrong and I can’t solve it.

I’ve been modifying the default kubrick template, and have replaced the “kubrickheader.jpg”, leaving the name the same, to make it as simple as possible. All I’m getting is a great big block of background colour. Wierder still is that this same block appears in *all* the other themes except one that I downloaded. I’ve literally replaced the entire theme with another that I *know* is working, giving it the same one that has the block, and still the ‘block’ shows up.
How can that be??

I’d be grateful if someone could steer me in the right direction.

Thanks

NIk