To further signal the trompe l'oeil status of your artwork, you could also make sure that no object in your painting is cut off by the picture's edge. Since such cropping can't have any real-world ...
According to legend, trompe l'oeil originated in a competition between artists Zeuxis and Parrhasius in ancient Greece. Zeuxis painted such lavishly appetizing grapes that birds tried to eat them.