Hello MrMidi! The Alpha does not work with the ac3d program, I see there also those pinks. But it works with the editors... I don't find any button from ac3d from where i can adjust the "pines0_a.txt"...
No, you won't.
All cautions about working with copies apply!
In GIMP or PS, export your bitmap. Then convert it to indexed color (aka 256 color mode).
Using the color tool in GIMP or PS, view the 256 color map, and locate the alpha color, and it's index number.
This is the number that should be in the <tree>_a.txt file, and it will change most anytime you edit your texture in GIMP or PS.