It's a best practice to pre-size certain images on your site to a specified size before uploading them, otherwise, you will eventually end up with aesthetic issues. For example, the featured slider images on your home page should all be sized to the same dimension to ensure a smooth transition from one image to the next. If the images are different sizes, then you may see white space under some images and different heights to images as they transition. Campus Suite does in fact create 12 different sized images based on the size of your original, but none of them may be the exact size needed for the banner area of your home page. Your designer (or Campus Suite support) can tell you the exact dimensions of your home page image sliders if you need them.
Conversely, if you're just dropping images into the content area of your site on a basic page, and you want to alight it right, and flow text around it, the auto-sized images are perfect. You only need to choose the size that is closest to that size when dropping in the image. GIF and PNG images should always be pre-cropped and you want to select the original image (that's the plus icon at he far-right of the image when you select it) that you uploaded.
Here is a helpful tutorial about how to crop slider images before you upload them to your site.