Q:

Why can you bypass restricted WiFis by adding "? .jpg" to the URL? 

I recently read an article on Hacking a commercial airport WLAN.  It's basically about circumventing paid airport WiFi redirections (they redirect you to a certain URL when you type something in the address bar). 
You just add ? .jpg and tada, you've done it.  
My question is: Why does this work?  

A:

Most probably the blocker is designed to let images through, maybe because they are hotlinking some images on the page where they ask for you to login. 
Appending ? .jpg to the URL makes the blocker think that the URL is an image.  On the other hand, anything after the ?  doesn't change the actual webpage requested, it only changes the GET headers. ( so http://google.com/?.jpg and http://google.com/ give the same page). 
Note that if there already is a query string in the URL (something after a question mark), then you must use &.jpg instead of ? .jpg, as only the first question mark is considered for making a query string (the second question mark is considered as part of the data, generally, and this may lead to you getting the wrong page)
Basically, when you fetch a page, you can provide additional data to the server (which can then send you a custom page dependant on your data).  There are two ways of doing this: a GET request and a POST request.  A GET request puts the data in the URL itself, a POST request sends in in a more subtle, 'hidden' way.  Here is an example GET request on the "ask question" page of SuperUser.  A note: it is not necessary that there is data when you use a GET request.  When you fetch a normal webpage, that is also a GET request, without data.