The Lightbox Javascript hasn't been updated since Sep. 2008. The Bug will only be fixed if someone else fixes it or the movie card uses a different javascript (or none) to display images.
I really don't know which problems you have with the script... For me all pictures appear scaled correctly! I have tested about 100 pics - front covers, back covers, DVDs and scenes from movies/tv shows. Something I must admit: sometimes a few pics are really showed mismatched, but showing them a second time they are okay again. In fact that behaviour occurs very rarely for me: about 3 or 4 percent maximum. I think that's nice to have - or?
In order to achieve what is possible, you have to try the impossible over and over again. Hermann Hesse
The javascript property's height and weight are unreliable read, because in the onload function it is not guaranteed that the image has already been loaded completely. This results in wrong values reported by javascript if the image is not completely loaded already.
You will see this "bug" sometimes with the new "HQ" Actor-Images on the Actors-Card.