Image Scaling in Layer Slider

I have set up a layer slider on my homepage (http://urbancraftuprising.com). It all seems to be working properly, except that I cannot for the life of me figure out how to scale the images! These are large images, but I was expecting there would be some option to set them to scale to the 600×300 dimensions I have set for my slider, so that the entire photo is visible when the slider is displayed.

This has to be an option somewhere, right? Please help me find it!