Background image in posts

The quickest way to achieve this effect is to position: absolute the background image. If your background image is in front of your text you can then use z-index to change the z-order.

See https://developer.mozilla.org/en-US/docs/Web/CSS/position and https://developer.mozilla.org/en-US/docs/Web/CSS/z-index for reference on the 2 mentioned properties.