This is only useful for wordpress.com users (I suppose), so read on if you’re one.
When I tried to setup a custom header, I edited the “default” one, provided by the Kubrick theme. This lead to an image with a gray border on top of it. It looked like the left side of this image (click for full-size):
Now it looks like the right side :) as it should.
As Mike, from WordPress support, so kindly explained, not many people edit the default picture. For this reason, WordPress doesn’t expect to find that gray border on top of the uploaded images (that’s why it puts it a bit lower). A few minutes of Gimp, and the new image is ready. Works on Firefox, Konqueror, Opera and IE6 (yuck!).
One more issue solved, and thank you again, WordPress!