Yes, the hash you see in transmission is a completely different one than the md5sum of the downloaded file. When the file is downloaded, run md5sum on it in a terminal.
Originally Posted by kstella
Something like this should tell you if your image is good.
It will check the hash of all four xubuntu-isos, so you should get a failure on the three you didn't download, and an OK on the one you did.
cd Desktop # or wherever you've downloaded the iso to
wget -q -O- http://cdimages.ubuntu.com/xubuntu/releases/hardy/release/MD5SUMS | md5sum -c