Translucent marble skins - how to convert?

I recently crafted and then tested out a simple PNG marble skin, and it appears invisible when marble reflections are turned on (basic or advanced). Then I looked at the old (MBP) custom marble skins in PQ's directory and noticed they are slightly translucent, allowing them to show up with or without marble reflections. I haven't been able to find an answer yet; how do you convert a plain, opaque PNG to be that way so that it is compatible with marble reflections in PQ?

Thanks very much

Open the image in an editor that has decent transparency/alpha support, lower the opacity of the layer, then save as a 32-bit PNG.
Paint.NET, GIMP, and Krita are examples of adequate image editors. You can also try using an online tool. For example, with this one , open the image, right-click the layer on the right, set the opacity with the slider, then export to PNG. No promises the online tools aren't sketchy.
The higher the opacity, the more reflective the marble will be. This is why opaque marble textures are completely reflective. It's counterintuitive, but that's how it works.

This works, thanks! I had to set the opacity really low, down to 25%. I wonder if this thing about marble reflectiveness versus opacity was intentional or not. (After all, it's been clear that PQ always has room to expand and improve...)