I don't understand the question. Assuming you mean the GUI obtained by double-clicking the JAR (or by running the JAR with -gui) then it already creates .png files by default.
When processed, a file "Test.puml" will change to display as e.g. "Test.png [Test.puml] (58 entities)". And it will have created a file Test.png alongside the Test.puml. Whenever Test.puml is modified, then Test.png is recreated automatically (while the gui is open). Which sounds like exactly what you're asking for...? I use it like this all the time for my offline editing, it's a great feature.
