This topic has come up recently, as well as a variety of times before.
Png images (without a background layer) used for decals seem to render darker than jpgs, or tgas. So to confirm for myself that there is a difference in using png file type, versus tga and jpg, I ran a test. I do not know if the difference in results are due to A:M, or the way photoshop handles pngs.
I took the original png image which had NO background, added a white background (layer), saved (in photoshop 6.0) as a new png. Then saved a copy in jpg format, and tga format. I did not do any adjustment to the image itself.