As an example in this case, say the law was changed and the penalty was a fine of $100 a day per violation. He could be fined for every day that the picture remained up after the law went into effect but could not be fined for days it was up prior to the law taking effect.
I was doing some research, and it appears ex post facto is supposed to apply to both civil and criminal law. Currently, precedent appears to say it only applies to criminal law, however, that appears to be an extremely flawed decision (Calder vs. Bull) by the Supreme Court.
But, if the law is changed, does his hanging of the photo become a criminal offense or a civil violation?
All that aside, as one poster pointed out, there are also free speech issues involved. Which, is probably the better argument.