Assuming you've already installed the shortcode (check Installation), you'll still need to activate the widget. Go to Appearance → Widgets, and move "Gracepoint Banner Widget" to the "Header" box.
Each "item" in the carousel assumes there are two separate images. If you look at the App Store (or iTunes), the thumbnail on the right is not simply the large image scaled down; it's a different image entirely. However, if you don't have a thumbed version, it's ok — the system is smart enough to scale down the large version.
The larger image is 835x345, whereas the smaller one is 115x265. We've created a nifty .psd file that you can use as a template (be sure to turn on guides).
At a2f Riverside, we're upload all our banner images to Flickr (into a set titled "Banner"). From here, we link all our files into Wordpress.
The order of GP Banner Widget is LIFO — last in, first out. That way, whenever you add a new item, it will show up as the first item. Use this knowledge as your weapon.