The simple answer is "you cannot do that". You might be able to put something on the server side that will check the referer before serving the image, but even that is not 100% guaranteed. Moreover, even if you did manage somehow to prevent this, nothing would prevent somebody from taking a screenshot of the browser page and then cropping the image out of it.
I think a much better approach would be to have a server-side url rewriting and processing of the images to add some sort of a visible watermark identifying the images as owned by you and saving a proper copyright information in the EXIF information.
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