There are many different ways that a work of art could reflect the influence of the ancient Greeks and Romans. First, a work might portray a scene from classical mythology or figures from antiquity. There are countless examples of this from the Italian Renaissance, but two in particular are The Birth of Venus by Sandro Botticelli and The School of Athens by Raphael.
Second, many works of classical art, particularly sculpture, portrayed an idealized human form. Classical sculptors like Myron, who is credited with The Discus Thrower, portrayed sleek, athletic, muscular bodies in their works as a way of celebrating the human form. This trend emerged again during the Renaissance, when sculptors like Michelangelo and painters like Leonardo da Vinci sought to portray perfection in the human form, with an even greater attention to detail than classical artists. Da Vinci, in particular, was fascinated by proportion, an interest of ancient Greek sculptors and architects as well.
Finally, a work of art might portray some perceived classical virtue didactically, in order to convey a message for viewers. A notable example of this practice can be found in the Neoclassical works of Jacque-Louis David, whose Oath of the Horatii, Brutus Receiving the Bodies of his Sons, and The Death of Socrates each depict heroic figures making enormous sacrifices for a cause. David, painting in the days leading to the Revolution (and during and after the Revolution, for that matter) sought to convey a message using classical stories that most viewers would have been familiar with.