Mustang, note that doing alpha transparency for png is fundamentally different than for tga images. With the latter you simply create a new channel called "alpha." While this technically works with png you end up with ugly white halos due to anti-aliasing and the fact that alpha-ed out portions of the image are turned white when the png is saved. Instead you need to set the image as a layer, eliminate all other layers (including the background layer,) add a mask to the image layer, and paste your alpha image onto the mask's channel.
Yup. I use .TGAs at work (or separate alpha images that the code compiles together) so working with .TGA and alpha channels is like "second nature" to me... PNGs gave me so much unneeded hassle and problems + different workflow that I just ditched PNG - why fight when I have working alternative? (bit like DarkBasic vs. Blitz3D... :)).