I suggest you use the CustomCSS as a testbed, and then move code into the child theme when you're happy with it. (So remove it from the CustomCSS - any code in the CustomCSS will override the child theme so avoid having conflicting code between them).
Child Theme sounds far more complicated than it actually is. Once you've done it, you'll find your understanding will give you more confidence to try other things.
To have your Static image on ALL posts & pages: it could be done with code, I'd suggest the easiest way would be to setup the same 1-image Slider on each Page using the Slider On/Off feature. If you have 100+ pages then that will be impractical.