Yeah I have the Windows 7 icons extracted from a dll.
This is what I have done, is that correct:
Original image: I selected the area with alpha values I wanted to replace later, then I opened the silhoutte plus plugin and selected these options:
All Alpha less than 255
R: 0
G: 0
B: 0
Both sliders are at the very bottom.
This would create a black image, which pixels on the right having more transparency (the exact same alpha values the original had).
Than I would copy that area to the clipboard.
The image I want to place in the original image (after having adjusted its size):
I selected everything, then I selected the alpha mask plugin and checked all boxes, although "mix alpha" would not change anything I could see. (This image had no transparency if that matters).
Then the image had transparency and I copied it to the original image.
It looks pretty OK althoug a little bit dark to me, is this the procedure you meant?