So if the initial request comes with "Accept: text/html" then you serve a webpage. The request when linked as img comes as "Accept: /" and you serve an image.

So if the initial request comes with "Accept: text/html" then you serve a webpage. The request when linked as img comes as "Accept: /" and you serve an image. Perhaps caching or other schemes would break this, I'll try to knock up a PoC.