It says, "Images of exactly 1000 × 388 pixels will be used as-is." When uploading an image of this size and then checking the website it looks like the pic is not displayed with 1000 x 388 pixels but is compressed somewhere to 800 x 310.
In order to have best quality with a header image it would be better to not have a big pic forced smaller by css code but to have the pic in exactly that size how it is displayed in the template header.
So I'm not sure what is the exact size how twenty eleven finally displays a custom header pic, but to have a 1000 × 388 compressed to somewhere like 800 x 310, causes the pic to be slightly unsharp, blurred. Thanks for any help!