Customizing 404 error page

I would like to change the text and add an image to the error page.

I found 404.php and made text changes, which didn’t work. Then I copied the page to my child folder, changed the “hueman” to “hueman-child” but that was a no-go, too.

I really don’t “know” php but follow directions pretty well. The information I’ve found on adding an image to a .php page aren’t working.

Any help would be greatly appreciated.