Use _Specular.png as the alpha channel in Photoshop

for paint dot net

When making a skin:
-Open a girl's base skin in paint.net
-Add nipples and pussy as layers
-merge all the layers
-Open the _Specular.png as a seperate file in paint.net
-Select the whole specular image and copy to clipboard (Ctrl + C)
-Return to the girl's skin document and select the merged skin layer
-Use Paste Alpha: Effects > Object > Paste Alpha (use default settings)

-Export as DDS: File > Save As > File Type dropdown > DirectDraw Surface...
 settings:
	BC7 (sRGB, DX 11+)
	-OK

finished
