If you consider this for some sort of security or privacy, forget it. There is no way you can stop the user from permanently keeping the picture.
Someone who knows the web can analyze the html and get&save the picture via script.
Someone who knows a bit less can disable Javascript and stop the refresh.
All others just take a screenshot.
^This, there's always a workaround. And anyone can take a screen-shot.
What's this about? If you're concerned about someone copying your Art-work or something like that, watermark and publish only smaller sizes for preview purpose only.