For transparency of background images a bit of custom CSS can do this quite easily I think.

Firstly, in the Background Begin includelet, add a name in the CSS Classes setting. Like 'transparent' or whatever. You will then use this in the custom CSS.

In your Raw Header HTML in the theme options (or in a custom CSS file), add something like:

<style>
.background-image-wrapper.transparent {
  opacity: 0.5;
}
</style>

Put your chosen css class name in the selector name after the second dot. And set the opacity to whatever you like. Any number between 0 (transparent) and 1 (opaque).

Thanks

Joe