CSS background images in WordPress

PHP code cannot run in .css file, however you can use inline style, such as:

<div style="background-image: url("<?php //url ?>");">

or

<style>
  .class-name {
    background-image: url("<?php //url ?>");
  }
</style>

The above would be useful when working with custom fields for dynamic image paths.

If the image is located in the theme directory, PHP won't be needed, let's say the image folder is directly under the theme folder /theme-name/images, and the stylesheet is /theme-name/style.css, then you can just add the background image to the css file as follows:

.class-name {
  background-image: url("images/file.jpg")
}

You don't need to use PHP in this question, your CSS file is already in template folder, so you can call image just like this:

background-image: url("images/parallax_image.jpg");

So you don't need to know template path to call images from your theme.

If the image folder is in the theme folder you can use:

background-image: url("/wp-content/themes/themeNameHere/images/parallax_image.jpg ");