I've recently discovered that someone is hotlinking to my website (displaying an image that's located on my website on their site by linking directly to the URL on my server). I'm going to change the .htaccess permissions so it redirects to another image informing the users that the image has attempted to be hotlinked. But is there any standard for how large this sort of image should be (in pixels), and what type (GIF, Jpeg) it should be?