[2005] Creating Transparent PNG Images

Hello,

I have an empty area where I can drag and drop pictures. Once I'm ready, I can save the picture (the area) to a transparent PNG on hard drive. Do you have any ideas how could I possible to that?

I'm creating a graphical editor and I need the final PNG to be transparent...

EDIT: Currently I have just basic gray background and a system where I can drag & drop pictures. I need to have now ability to save the final result to a transparent PNG.

No one knows :(