如何扩展Twitter Bootstrap图像轮播以获得动态大小的居中图像

I'm trying to extend the default Bootstrap image carousel to support dynamically sized images (500x400 max), centered both horizontally and vertically. Additionally, I'd like to maintain the original caption layout, which anchors the caption to the bottom of the image, with the caption div extending fully across the image (but no further.)I'm trying to extend the default Bootstrap imag