Some notes:

• 'opendir' should check its result, perhaps with a "|| die"
• 'srand' should not be in the subroutine, since it could be called more than once in the same script.
• The whole subroutine could probably be collapsed into an inner loop.
• If the directory is empty, the script will enter an infinite loop!
• Outputing a 'Content-size' to the browser would be nice.
• This does not actually return the image to the HTML page, but a location. Strange, but workable for most browsers.