Sure it is possible.

The trick #1, redirecting everything to some cute site is doable out of the box, of course on N9 you need to be on open mode to change iptables.

The trick #2, manipulating images on the fly takes a bit more. You would need at least imagemagic and squid on your device, of which I think neither have been ported.
(But as a proof-of concept I think you could run them on Easy Debian, that should work OK...?)