Simple Uploading Images Problem

First time using WordPress.

Trying to change the header image on Twentyeleven theme.

When I ‘Choose File’ and then ‘Upload’ I get no image to crop (small blue square with question mark). Tried all different sizes including exact size and file types .jpg .png. No joy.

I have also tried uploading an image using ‘Add New’ in Media tab and although the image appears in the Media Library, again I get the blue question mark error instead of a thumbnail.

I have also tried uploading directly to the wp-content/uploads folder on the server via FTP and also to the wp-content/themes/twentyeleven/images/headers folder but these don’t appear in the Media Library.

Thanks