this is called a captive portal. you need something a bit more... advanced than a router. apparently, pfsense can do it though.
https://en.wikipedia.org/wiki/Captive_portal
Long ago I had the same idea and found that it was possible to direct router traffic through a pc. At least I am pretty sure. Then you can do whatever you want (flip or replace images etc).
No links or references, sorry.
Btw why is this in code?