If your CSS file is located in the folder
/wp-content/themes/portfolio-press-child/css and your images are in
/wp-content/themes/portfolio-press-child/images, then you should be able to simply give that background image to an element by using this:
If it doesn’t show up, check if the image can be accessed directly by going straight to http://yourwebsiteurl/wp-content/themes/portfolio-press-child/images/my_image.png
If not, we’d have to see a bit more of your site to see what could be wrong.