"Images on the internet are 'free game' - if someone posted I should be able to use it."

Yes and no.  Yes because you put it you put it on the internet.  Also if anybody can view it there isn't really a big difference if they use it for a project.  No because you did the work, you went out there and took the picture.  And if somebody else wants that picture then they can take it themselves.  I am leaning a little more towards yes though.  Because if you didn't want anybody to use it then you shouldn't have put it on the internet in the first place.