header image and menu fixed to top with content scrolling over background image

Hello everyone,
Can someone please offer some advice or assistance on this….
I want to have a main background image, and have the page content scroll over the top of image but be somewhat transparent, so you can still see the background image.

I think i have accomplished the back image and scrolling part, but need a few tweaks.
I want the navigation to be fixed to top of page. then a logo centered underneath and then page content, all flowing over the background image.

My problem, one of a few so far, is that if i scroll up the background image isnt as opaque as when i scroll down. I want it to be fixed opaque, and have menu fixed at top, then logo and all page content to scroll overtop of logo.

I hope this makes sense. Go to link please and scroll up…you will see the image in full color. ascroll down and it goes opaque or semi transparent and then content flows over the back image.

How can I fix so back image stays at top and doesnt scroll full color and keep menu fixed at top of page?

Thank you in advance for any help with this. Perhaps i have the wrong theme?

The page I need help with: [log in to see the link]