this seems as a trivial question but after a couple of hours fiddling with the Twitter Bootstrap carousel in their example (http://twitter.github.io/bootstrap/examples/carou
You can simply change their css to this but it will cause your image to crop of what doesn't fit:
.carousel-inner > .item > img { position: absolute; top: 0; left: 0; min-width: 100%; min-height: 500px; /* <--- Change this line to min-height */ }