A simple plugin that uses randomly picks a header image from the files in wp-content/header-images. Install, activate, that’s it.
Put only image files in wp-content/header-images. The plugin will choose any files from that directory, so other file types will mess things up.
The plugin only sets the image url, not the size, so all images should be the same size and the same size as the standard image header for your theme. That’s 1000×288 for twentyeleven or 940×198 for twentyten.
The plugin does not deactivate the custom header image functions of any themes. So your theme might still allow you to set the image header, but this plugin will most likely override it.
If you cannot make this plugin work, please try another. I’m not willing to provide support for this plugin. Please respect that decision and do not ask support questions of me. If you find a bug, or the plugin does something you think it shouldn’t, please let me know in the comments here. Also, if you’d like to take over maintenance of this plugin and provide support, let me know.
Get the code on wordpress.org.