You want to use that high definition transparent PNG in the header of your site but whatever you can do it can’t be smaller than 2,5Mb?
Use the SVGIZE tool to transform your PNG24 into a JPEG+8but PNG Alpha inside of a SVG file. You will reduce your file size by 75% or so.
This tool was made by Vlad Danilov on my request and I’ve added an Automator Workflow for macOS so you can use it as a service in the finder with just a right click.
Download the tool and the workflow from here: