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.
?.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/ 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.