It's really impossible to separate good and bad are from your own subjective opinion, as there is no true objective criteria that you can judge art by. There are criteria that can be chosen -- for example, how did the artist apply light to their painting -- but these criteria 1. depend on the specific kind of art (in the same vain, not all paintings utilize light in the same way or for the same purpose) and 2. are inherently subjective because they are being chosen by a subjective being.

The closest we can maybe come to being able to objectively judging art is as a collective/society saying that some art is good, and that the opinion holds true over time.