There are several methods to create alpha channels. Here is the easier and quicker one (but not the best for cars textures):
1 - Create a new file with the same dimensions than your original texture.
2- click on your texture, then Selection/ select all
3- move the texture into the new window.
4 - click on the original texture window and click on the "layers" tab: you see 4 channels: RVB, red, green and blue
5- On the bottom of the layers tab, click on "create a new layer" button (the window appears in black. Deselect the RVB, red, green and blue channels if they are enabled.
6- Click on the new window with the copy of your original texture, Selection/select all
7 - move the copy into the alpha channel (original texture window)
8 - Click on Edition /Fill with grey colour, opacity 50%. (You can also modify the luminosity and contrast if you want it a bit darker)
Save. It is done.
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