How do I get an image to stay centered when its fluid width (percentage based) container is too small to show the whole image?
If you are anticipating that the image to be displayed will be so much longer than the display container, setting left: 100%; and margin-left: -200%; (from Bryce's answer) might not be enough to get the center part of the image. Just put a bigger percentage for both. Make sure that the other is half of the other though.
left: 500%;
margin-left: -1000%;