Here’s a few things to check
1) Are you using Firebug to create and check your css. It’s great and will give you to correct elements to target.
2) This will let you check your child theme is working ok
3) You can upload your images via your media library in your wp dashboard. Once uploaded it will present the full url to each image (so that you can target in your css if needed).
Let me know if that makes sense :)