1. In general, yes. You are harming those who would have otherwise enjoyed and benefitted from the painting. And you are harming the painter, who invested part of their life in the creation.
2. In this case, you are only harming yourself. If you hate the painting, destroy it.
3. Here only the first harm of 1. applies, not necessarily the second.

Morality can be objective as you like, presuming you accept axoims: i.e. harming others is bad. In this way it is as objective as math.