In response to
      body {
        background: url('black_alpha_25_percent.png'), url('bright-and-shiny.jpg');

This demonstrates stacking two background images. The image on top is a black png set to 25% opacity, which results in a subdued version of the bright-and-shiny.jpg underneath it.

Notice the comma, this is one line. But the stacking order is important. The first one will be on top, the next underneath that. If you had a 3rd or more, they would be proceed to be underneath the one before it.

For the sake of this example, I only covered the left 600 px so you could see the contrast between the normal and subdued backgrounds.