This answer is tangential because it is about U.S. law and not Brazilian law, but maybe it will be helpful.
You can get an exhaustive list of court cases on U.S. "fair use" law here: http://copyright.gov/fair-use/fair-index.html
I browsed through them trying to find something directly relevant, but I was rather surprised to find that I couldn't find quite the case you're talking about. Maybe this is considered so obviously settled law that it hasn't come to court.
In general, criteria for qualifying for "fair use" are:
Did your copying hurt the market for the original work? That is, is your copying such that people are likely to buy your copy rather than the original? (Remember, copyright is all about PROPERTY rights, your right to make money from your property. People often have the mistaken idea that it is about protecting your reputation as the author of the work and that sort of thing. No.)
How much of the work was copied? If you copy one sentence from a 300-page novel, you're on much firmer footing than if you copy 299 pages.
Was the use "transformative"? That is, did you just copy what somebody else did, or have you "transformed" it in some way, such as using it for a purpose different from the original purpose, modifying it, parodying it, criticizing it, etc?
Was the use for purpose of education, research, or criticism? Quoting someone else as part of a literary review or a movie review is given wider latitude than simply incorporating someone else's work into your own.
I am not a lawyer, but I'd think that including a screen grab as part of a review would be about as safe as it gets on a fair use defense. The cases I read through included several where someone made a documentary about the history of film and included scenes from movies that they wanted to comment on, and these were consistently ruled fair use. Another was someone using footage of a boxing match in a biography about the boxer, again ruled fair use.