Hi Norm, I love screencast-o-matic, that’s what I use for my screencasts. It’s simple, cheap, and you can record videos longer than 5 minutes which is awesome!
The size options in the media uploader are for adding the image as content to a page or post. When you add it as a featured image, you don’t have to choose a size, that is automated by WordPress.
I checked the site you are working on, and the slide images are indeed 600×450. If you right-click on one and view the image itself, it is the full size, not square.
The trick is with your theme, the main content column you are adding the images to is 500px wide, and it is setup to hide anything beyond that width so it doesn’t stick out of the column.
So for this theme, I would set you slideshow width to a max of 500px, and change the height accordingly.