I recognize that many climate activists are doing things I would never do and believe are strategically wrong. As you note, most of the actions that "damage paintings" do not damage them: they are poured harmlessly on the protective glass.
I do not think these are generally useful tactics. But I also recognize the desperation and fear that motivates those tactics. And I respect the courage and sincerity of those who are willing to go to prison for their cause. OUR cause, because all of us live on Earth and depend on the same ecosphere.
That said, I am part of a group with a different approach. We are new and small (about 300 members and something like 2,000 followers) and—so far—nearly invisible. We aim to be a non-violent but very effective movement, inspired by Gandhi and King and, more recently, the Serbs of Otpor! that non-violently removed their dictator Milosevic.
We intend to use imagination, planning, humor and a variety of actions to (ultimately) force the issue. Our goal is to establish a fully-empowered worldwide emergency program, led by qualified climate scientists, to seriously address the crisis.
We believe it is literally insane to destroy the planet we all live on. We are The Saners.
Read about us here: WeAreSaners.org
You can sign up for our email list at WeAreSaners.org/join.