Block direct access of a page through browser.

Hi,
I have a situation where I have a page "image.aspx" which displays an image from a stream using Response.OutputStream. Other pages make use of this page by calling "image.aspx" in <img src="image.aspx" />.
Now, the issue is that if a user directly access the page "image.aspx" in a browser, he would be able to see the image. I would like to restrict such access. It would be great if you can let me know how I can make this possible. I would like to acheive this without using Session.
 
Thanks in advance.
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