Celestia Forums

Download your 4K map in JPG format from here.

Start up PSP.
Load the cloud map.

In the menu, select Mask / New / From Image
Source window: This window
Click OK.

You now see that all black in the image has been replaced with transparency.
Now save the image as a PNG file, and place it in your textures folder.

--- edit ---

Well PSP7 is a rather old program but for basic image editing it works fine.

My best guess is that is converts black to perfect tranparent and white to perfect white. Anything in between is partially transparent.

If I hide the transparency the image is just the same as the original JPG.

What I do when getting cloud maps is, before adding transparency, I adjust the brightness and contrast, so I can get large sections of cloud completely opaque. Increasing the contrast usually helps.