Because I am trying to install from a USB stick, so naturally, when I went to the Download section and saw a USB Image as one of the two choices for downloading, I chose the USB Image. Seems logical to me! Going back to the installation instructions for using a USB stick, I then saw that it says "download the ISO image" so I went back and downloaded the 3.12 .iso image.

Edit3: Finally got the installation going. I had to "Mount" the iso image, to be able to copy the "boot.img" file from the mac_ppc32 directory. Just double clicking the iso image as instructed on the page for USB installation, did not work for me.