I found a bug. I installed WordPress 3.5 with the Twenty Twelve theme. Then I went to Appearence, Header to upload some images to use as a header. It worked perfectly.
I then decided to install this plugin, the latest version 0.9.2.
After the plugin installation and activation, when I go to Appearance - Header, to upload new header images, it doesn't work :-( The uploaded images have their path incorrect, and don't display in the browser.