Well my friend, there is no difference between the alpha channel of a TGA and the alpha channel of a PNG. Both are only different file formats. The content is the same. I assume that you are not familiar with the concept of transparent images and alpha channels, so check out the lite-c workshops. Theres a lesson about how to paint transparent images.

In other words, when your TGA image looks like sh**t your PNG image won't look any better.

In other words, learn to know what you're talking about and then you can avoid similar embarassing situations in the future. Hope this helps...