All of the jQuery image sliders I've seen require images to be the same size.
Do you know any which can dynamically resize their display pane to fit the size of the image. I have seen this done on commercial websites.
I was looking for the same thing and came across this... http://caroufredsel.frebsite.nl/examples/variable-visible-variable-size.php - it was the easiest to use and fully customizable.
Not sure if you found what you were looking for already, but just in case someone else is looking for the same thing - it may help.
Here is a slider which has auto-height capability and auto-width
This also has auto-height capability http://slidesjs.com/