With the Nodes addon installed, you'll go to Forums -> Node Layout & Styling, select the node you want to add the background to, and then check the "Use custom value" for "Background image". You'll then want to put the URL to where the image is stored in that field (so you'll need to upload it to your web server or some image hosting service, or upload it via XF's content management if you're on XF Cloud).
Once that's done you'll want to make sure node styling is enabled for your prefered style(s) (you can just select the style in question you want to enable styling on in the dropdown on that page where it lists out the nodes for styling, and then click the "Enable styling for style" button at the top).