UGh! I'm using the same host (anhosting) and process as all my other installs, I've been doing this for years. Images seems to be an ongoing issue with a lot of people.
When I went to the themes page the FIRST time to check out themes and maybe install a new one, all the images (screenshots, etc) showed the no-image icon from the very start.
I figured I would try anyway. Uploaded a .png for header.
Instead of showing the header image, my site is showing a GIANT header placeholder with the no-icon image. Its a huge square, when my image is 1200x100 px, long, like a banner.
I came here to read about it....
I deleted ALL plugins.
I refreshed and dumped my cache.
I installed twenty12.
I'm having a similar issue on an install I did a couple of weeks ago, and I gave up because I couldn't fix it and ended up just removing all images from the site- which is a dumb fix, we need images. So I'd like to figure this out.