I'm getting my but kicked here. I am trying to modify my OSEXT Theme to use PNGs that are modified and somewhat transparent. when i load a png from x-theme blue/gray, i find that they are transparent ~ 80%. I set up the layer for my modified left/right-corners/top-bottom pngs to the same and save as PNG24 with the transparency turned on. and no dice
It's quite simple... alpha channel is like gray scale channel, where level of 'grayness' represents level of transparency. You can use any tool on the alpha channel, for example you can draw, select, fill, etc on this channel, and the final image will be more or less transparent.
For start, you can create layer mask, paint it half black, half white, and see what it looks like.