Well, we could start with the fact that you have no user added content on the default twentytwelve theme. You actually installed this latest version of WordPress very recently (sometime yesterday ?)
There is not one single request for a file possessing and image file extension in the server responses to your front page.
The path to the logo that shows above your login dialog at /wp-admin is just fine...
..so we'll assume there is no problem with file ownership or permissions schemes - for now - that are preventing directory or image file access.
The path to your uploads directory results in a 404.
This indicates that you probably haven't uploaded any images. If you had, I would probably get either a 403 error or a directory listing of the /uploads contents, depending on whether or not you prevent casual directory browsing.
- Switch to a different theme that has a header image. See if it shows.
- Create a post with some content and insert an image. See if is shows.
- Or, navigate to Dashboard > Appearance > Header and upload a header and see if it shows in the twentytwelve theme.
Right now, that theme is operating to design intent. I see no indication anywhere on your site or in your server responses that images are being prevented from showing. If you would like to try and add some image content, I would be happy to take another look later.