Transparent PNG Image

[Victor Odhner]

The PLUG web site is using some images from the Plug Presents
web site:
  http://www.plugpresents.com/images/unitywavelogo.jpg
  http://www.plugpresents.com/images/copyleft.png

These appear against a white background on the Plug Presents
site, but the PLUG site ( http://plug.phoenix.az.us/ ) has
an image as a background.

How can I convert these two .PNG images to be transparent?
If you know, please just point me at the tool in question
and give me a pointer or two, to save me some learning curve.

Thanks,

Vic