I am wanting to know how to work with a piece of black and white line art to make the white transparent. The problem if I just simply select the white space, with a tolerance of, say 20, it leaves the "black" parts of the image still in greyscale. What I need to be able to do is convert that black-white gradient into a black-transparent gradient. So that there is a gradient of partial transparency that avoids harsh edges in the finished image. (The intention then is to convert it into an alpha channel).

The only online tutorials I can find talk about Photoshop and the techniques don't seem to work in PSP. How can I achieve this in PSP X?