I have an iMac G4 onto which I am trying to install Ubuntu 12.4.1. But every time I download the image, I get a message warning that the image might be damaged. Is this a problem with the image or with OSX? If it's a problem with the image, is there an image out there that isn't damaged that will run on an iMac G4 PowerPC?
Try one of the Ubuntu mirrors for example: http://linuxfreedom.com/ubuntu-releases/12.04.1/
There's good list of Ubuntu mirrors here: https://launchpad.net/ubuntu/+cdmirrors
You'll notice there's also a file available in the download directory called MD5SUMS. You should check your downloaded file against the MS5sum for that iso in the MD5sum file. To get the MD5sum of the downloaded file simply type
replacing the filename with the correct name of the file you downloaded.
Using a torrent instead of a direct download is a good way of getting one of the distribution files if you are having corruption problems downloading it normally.