Here are tons more additions to The Best Sites To Learn About Mardi Gras & Carnivals:
Rio Carnival 2012: costumes and celebrations is a slideshow from The Guardian.
In pictures: Rio de Janeiro carnival is from The BBC.
Rio de Janeiro Carnival 2012: the first night of parades in the Sambadrome is a slideshow from The Telegraph.
Here are several CNN videos:
Carnival Has Started in Brazil:
Celebrating Carnival in Trinidad: