For the bottom inset background, that is my fault. I didn’t realize I left that option setting in there. That is left over from when I first made the theme and was planning something for that position but did not follow through with it. Thanks for pointing that out.
For the banner image question of size…I generally say as wide as you can do it, preferably not less than 1920 pixels for those with wider screens (2560 pixels if you want to include the REALLY wide screens on 27″ monitors). Height can be any height you want. As the window downsizes, the image will be responsive and adjust as the viewing screen gets smaller.
As for the child theme, never too late but please note when you activate it, WordPress will see it as a new theme activation and you would need to redo your theme option settings.
What colour is the blue of your page you were talking about before and needing the curve that colour?