How are idents like these made? Here is a YouTube link to a Channel 9 ident from Australia featuring characters from "The Big Bang Theory": 
                
            

I was watching it and it made me think about a few things:


Do the Nine Network have to get permission from CBS/Warner Bros. for the use of "The Big Bang Theory" in their ident?
Described simply, how do video editors go about featuring the characters so smoothly in a pre-made ident? Obviously it can't have been done on a greenscreen since the characters are all the way over in the US, so how could this be done using standard footage from the show?
Yes, they need permission. However, the permission to use the characters in image and other promotional material is likely included in the international distribution or syndication agreement.
It almost definitely was done using a chroma key or other intentional compositing technique, as opposed to trying to mask the movements out of footage from the show. Networks, syndicators, and international distributors usually supply their affiliates with promotional materials. Often, these promotional materials include elements that can be built up into a complete promo that fits the affiliate's branding. The distributor will provide suggestions or instructions on just how a particular element should be used, but it is up to the affiliate to put in their own look.