Prevent direct access to image files?

Lets say I have a folder named Pictures under my domain  (mydomain.com/pictures). I use these pictures on certain pages like with tags like:
<asp:Image ID="Image1" ImageUrl="pictures/image.jpg" runat="server" />
Now can I somehow prevent these pictures from being reached directly? I dont want to be able to access the picture by typing "mydomain.com/pictures/image.jpg" in the addressbar. I only want them accessed when called from server tags like the one above.





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