Well I decided not to be lazy and share the way I do it
First of all I work in Photoshop, don't know how to do it in other apps, as I never tried it...
So let's say you made a nice shoreline tile for your new terrain/repaint, but now you have to make alpha channel for it...
So once you opened your image in photoshop...
...on your "Layers" toolbar go to channels and duplicate one of them, I usually choose "the tail end Charlie" as a true virtual fighter pilot and that is Blue channel
After that, selecting only the channel you just created go(on your main toolbar) to Image -> Adjustments -> Threshold...
A window will pop out, so adjust the image with the slider shown on the picture until you get something similar to the one you can see on the screenshot. It will rarely if ever turn out perfectly so clean it up as needed with the brush...
Now make other RGB channels visible and clean up the image with fine brush tip just to smooth it out and clearly define the shoreline so you don't get bad effects in game:
End result looks something like this:
Now that's a nice alpha channel with just a few minutes of work :yes:
In the end just don't forget to save the *.TGA with alpha channels:
This is how I do it, I hope it can help somebody