Will I get in trouble for using a picture of a celebrity on a advertisment poster?

If you were actually using Barry Manilow's likeness for a commercial purpose without his permission, you could be liable to to pay him damages.  In most jurisdictions, celebrity likenesses are like trademarks.  Just as you can't use an adidas or ralph lauren trademark on your goods without permission, you can't use Barry Manilow's picture on your drink either.  However, since you actually used the picture for a class project, and not for commercial purposes, you should be ok.