How does Henry David Thoreau use imagery in Walden?

Imagery:

“If you have built castles in the air, your work need not be lost; that is where they should be. Now put the foundations under them.”

“Live in each season as it passes; breathe the air, drink the drink, taste the fruit, and resign yourself to the influence of the earth.”

"The setting sun is reflected from the windows of the almshouse as brightly as from the rich man's abode; the snow melts before its door as early in the spring. I do not see but a quiet mind may live as contentedly there, and have as cheering thoughts, as in a palace.”