Broken images

When installing a new WP-site, the images is not shown, not even graphics in the admin panel as icons is tinymce or the wp-logo. When right-clicking on the placeholder of an image and selecting “View Image”, the image is shown (the path is correct). After going back in the browser, the picture is shown on the page, but after reloading the page you only see the placeholder again.
I have tested many different versions (2.0.11 and 2.3 and 2.3.1) of WP with the same result. All my “old” WP sites are okey.
I have also tested both Fantastico install and a “clean” manual install with the same bad result.

Any ideas?