The link Panos posted is to a post on his blog that contains a list all of the themes that do have the "hide text" feature. This is a link to the list of all themes that have customizable image headers > http://en.support.wordpress.com/themes/custom-header-image/ Only a few of the new themes have the option of uploading a background image into them.
You can use the search utility and Feature Filters in your Dashboard > Appearance > Browse Themes > Feature Filters to locate themes of interest to you. (A-Z, Popular, Features)
Each theme has a thumbshot and a features summary description. You can browse and when you find one of interest you can click “Preview”. When you are ready to make your selection you can click “Activate”.