how do I get the transparencies in PNG images to show up in renders?

I modeled hair instead of using particles, and I used a PNG images i painted in photoshop of hair, but when I render the picture, the parts on the image that should be transparent, arent

When you load the image in a texture channel, make sure you activate Use Alpha in the Image panel buttons. If you just want to layer this texture on top of another one this is enough. If you’re trying to make a plane transparent except where you painted the hair, Gabio wrote a tutorial on this ‘card’ technique here.