I wonder what the chances are that a simple painting like this (a few large rectangular blocks) is a valid program. From skimming the documentation, I tend to think even that every such image would run without error, given that "Any operations which cannot be performed (such as popping values when not enough are on the stack) are simply ignored, and processing continues with the next command."

However, another question is how many of such random images would actually do something "meaningful".