How do I disable JavaScript and CSS render blocking

I meant on the front end of mine, and several of my clients, sites’. I am frequently hired to optimize the sites, among other things, and I know for a fact that unless you optimize the image, your getting an unoptimized image. You can use tricks to fix it, or just optimize it, but throw a 4mb pic in your template and view it live, it’s 4mb. I can get that same 4mb image down to 56kb.