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Hi!
It does not come from Gimp, I use it for my sprites that are not fully opaque and it works, I use almost only PNG.
When I view your demo, it looks like the background texture is filled with garbage data (a bunch of black and white snow, with color spots and rectangular shapes). If this is what you see when you try your alternate texture, I would suspect that an error occurred when you were creating the texture, causing the glTexImage() command to fail and leave your texture filled with bogus data.
One possibility is that your textures were different sizes and you forgot to update the coordinates? I would recommend using the DebugGL or checking for errors before and after you initialize your textures.
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Unfortunately, I still get the garbage 
I know I’ve tried your pilot demo before without any problems (maybe last week?). I’m on a Macbook Pro 10.5.7 with an Nvidia 8600MT(?).
Anyway, here’s the screen capture (the swirly bits were from a screen-saver … the garbage):
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The good news is that I tested your demo and everything works fine. Unfortunately I couldn’t tell where there were blending problems.
I noticed that you’re using the REPLACE env mode. With the REPLACE mode and an RGBA texture (which is the case for png images I think), the final alpha is taken from the texture and not the color. Since most of your texture would have an alpha of 1, it explains why it appears as if you used GLColor4f(1, 1, 1, 1).
I would recommend experimenting with DECAL and MODULATE to see if they do what you want. If they still don’t work, there is some advanced texture environment blending by using the COMBINE env mode. That has a lot of details and I think it’s best to try the others first.
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