I had a very simple program that did this ages ago. It had thumb in the name and I just can't find it.
I have no idea how it works, but I found on the web a free program called GIMP:
GIMP - Downloads
According to a message board for it, this program contains a tool to mass edit from jpg to png.
Also, I have infranview at home and will poke around on it this wekend to see if I can find the feature.