From the news story:
Brennan thinks he committed no crime.
"I know that 'nude, but not lewd,' is protected speech in Oregon," he said.
According to ORS 163.465.
(1) A person commits the crime of public indecency if while in, or in view of, a public place the person performs:
(a) An act of sexual intercourse;
(b) An act of deviate sexual intercourse; or
(c) An act of exposing the genitals of the person with the intent of arousing the sexual desire of the person or another person.
Based on the picture, I think it would be a tough sell for a jury to prove that his intent was "arousing sexual desire."