That is a tremendous honour for you, having your artwork stolen by a nation rather than merely being reposted on other people’s blogs will mean that your work will be seen by many more people than ever before (I am assuming that the government of Brazil did not bother to contact you regarding the appropriate licensing fees for the publication of your work … their assumption being that you would never find out about the billboard [damn Internet]).
I know that composers have some protection against this sort of thing through the international performing rights groups that watch out for each other. As a member of the Canadian group, SOCAN – the Society of Composers, Authors and Music Publishers – when my music is performed in a foreign country I receive royalties from the PRG from the host country – though it can take over a year for the reporting to get back to SOCAN. Is there anything like this for cartoonists, illustrators and political cartoonists? If not, there should be – having your work stolen is not a good thing.