This metaphor means that rather than simply painting a scene, it is far more difficult and rewarding to paint the atmosphere or lens through which we see this picture. This metaphor can be applied to what Thoreau writes about life; rather than simply going through our days with select times that we devote to contemplation or beauty, Thoreau thinks that we should try for this type of moral and spiritual elevation and transcendence at all times. This type of transcendence can be compared to painting the lens through which we see the world around us. As he writes, "To affect the quality of the day, that is the highest of arts." In other words, it is an art in and of itself to apply spiritual understanding and try to achieve beauty and inspiration from the world around us at all times. Thoreau compares this type of awareness to being awake, rather than spiritually slumbering through our days. 

Thoreau is using the metaphor of the artist in this quote.  He is explaining that the artist can carve and paint a scene to make something look beautiful, such as a bowl of fruit.  He is comparing that process to humanity and the human perspective.  He is saying that the way we look at things can be "carved and painted" by our perception and our attitude.  Things will look beautiful we we look at them with beauty in our mind and heart.

He takes this idea a little further in order to promote morality.  He says that if we are morally righteous - meaning free from sin - then our perception will be clean.  Thus, the world around us with be clean and "glorious".

"Every man is tasked to make his life, even in its details, worthy of contemplation of his most elevated and critical hour."

The details of the world we carve out for ourselves should be representative of our ideals.