set title of image document to display in browsers

The only possible way is not to open image directly in the browser's window, but a simple HTML code like this:

<html>
<head>
<title>
Put your title here
</title>
</head>
<body>
<img src='your_script_generating_images.php?parameters'>
</body>
</html>

But did you think about some fancy and nice solutions like http://highslide.com/ or http://fancybox.net/? All that you need is to set the links to your script generating images and to update the page title correspondingly (which is not even required as you can write information you want just above the image in popup).