Broken header image

Hi.

I’m using WordPress 3.3.1 and the theme Mantra 1.8.0.
What i’m trying to do is to uplead a custom header image. I’ve tried in both .jpg and .png. the resolution of the picture is 1050×300 pixels.

But when I upload the pic nothing shows but a small icon in the center of the pic like it’s broken. And when I visit the site, there’s nothing.

What should do?

I’ve tried to search for help but nothing helps.