I think that because existentialism, as a philosophy, is presented as more of an attitude than a formal argument, it more easily lends itself to be portrayed in films or other forms of art. In another one of my classes, we have been talking about Dadaism, and I think that this could be a form of existentialist art. Here is a site that has a lot of examples of Dadaist art.
Dadaism was a challenge because it was seemingly so meaningless. Before Dadaism, art was judge based on values such as the skill of the painter (think Michelangelo’s Sistine Chapel) or the genius of his or her ideas (like Impressionism’s new interpretation of how we see) or the ability of the art work to appear beautiful to us. Each of these values seems to be intrinsically precious: we do not choose what genius is, we merely find genius in the world and recognize it as intrinsically valuable.
To challenge these ideas, Dadaism created art that anyone could accomplish and the ideas behind them seemed more like random stupidity than genius. They made poems where words were chosen at random to create each line. Duchamp, a Dadaist, created the idea of ready-made art which is when the artist would choose a random object (a bottle rack or a urinal, for example), and then put it on display in a gallery.
Dadaism was largely a reaction to World War I. The artists were creating meaningless art to protest against the meaningless death and destruction that the war brought. Although we have seemingly rational reasoning behind our wars and ideas, so much death and suffering for the sake of keeping treaties and promises seems blatantly ridiculous. Dadaists were creating art that was just as senseless as the war but without rationalizing it and without pretending that there was anything intrinsically good or beautiful about their work. In this way, they reinforced the existentialist idea of human's constantly determining the meaning of things and events.
These acts of creating nonsense poems or ready made sculpture seem meaningless. I think that they are existential acts in that they ask the viewer and society to take them seriously and give them meaning or see the meaning or intention of the artist. We value and view Dadaist works as art despite the fact that they defy the basis upon which we defined and still define art from earlier in history.