Transparant pixels in a TPngimage object

Question

Is there anybody who can help me making pixels of an png picture transparant, such like: MakePixelTransparant(const png:TPngImage; x,y:integer; perc:integer); and what else do I have to do to makes thinks works (with values do i have to change to make the png object transparant, so i can use it in for example PhotoShop).

Thanks, John Duinmeijer.