I assume it has to do with how some wifi hotspot portals work. They redirect all request for web pages to the sign-in page until you've signed in. I assume that some of them don't redirect requests for images (for whatever reason) so there's a loophole you can use.

When you have a '?' in a URL, what follows supplies information to the website - but if the website isn't expecting any information it will just ignore it. So if you put in '?.jpg' at the end, it gets ignored by the website, but the theory is that the wifi portal sees the request as a request for an image file, not for a whole website, and simply by-pass the sign-in process.

I doubt many wifi providers work this way these days, but you never know.

(Just tried it with a BT Openzone hotspot visible from my office. It didn't work...)