The reason this happens is because the default Terran race.xml points to movies named RaceImage_Jenna*.bik instead of Jenna*.bik. So this is all just a sloppy oversight. There are similar problems with some of the minor races based on major ones, which are also inconsistently named.
To fix it go to your ...\GalCiv2Ultimate\Twilight\Movies directory and rename the files:
I checked the GalCiv 2 Unlimited Twilight RaceConfig.xml file and the references were:
<Race Name="Terran Alliance">
<Description>The humans recently joined the interstellar club with the invention of Hyperdrive. They are the diplomats and traders of the galaxy. Though a bit impulsive, they are tougher than they look.</Description>
So, it doesn't seem like a good idea to re-name those .bik files!
Using GalCiv 2 v2.04.017 direct from Stardock btw.