One of the key features of Zen (as a Buddhism or philosophy) is freedom, becoming free from almost everything, especially from fixed ways of thinking, experiences, traditions, even beliefs.
A painting is a very good medium to deliver some Zen concept and idea since a painting is a visual message, in a more abstract way than a poem. A poem delivers a message, some kind of thought, feeling or mood but by using words. By using words the message becomes more descriptive. A painting is more abstract. Abstract means more freedom, less restrictions. Viewers have more freedom how to see a painting and what to see in a painting than poem readers have when they read a poem.
Zen paintings look very simple in terms of objects, shapes of objects, compositions, colors. But simpleness does not mean a simple message. Viewers' interpretations of one simple Zen painting are diverse as viewers have a big freedom how to see and interpret it due to its simplicity. Zen paintings are usually very simple like depicting a simple real world object or a few objects and not so abstract - not necessarily abstract to give viewers a lot of freedom.
Many people say that abstract paintings are difficult to understand. Some people like and appreciate and enjoy abstract paintings mostly due to the freedom for understanding the paintings while some people do not like them or have no comment to give as they do not understand the paintings.
Zen paintings feature being simple and not so abstract but give viewers a lot of freedom
including the sense of freedom and the importance of freedom.
AAG