Header image not displaying after uploading

I’m using the twenty twelve wordpress theme. I have attempted to upload a new header image to a front, static page. After I upload the image it does not display in the thumbnail in the appearance>header window. Also, when I try to preview the site by clicking “visit site” (after saving the image and publishing the new page/s) from the main toolbar a white screen displays showing no content.
Am I missing a step somewhere?
Thanks.