It's a best practice to size key images on your site properly before uploading, otherwise, you will end up with aesthetic and quality issues eventually. For example, the featured banners on your home page all need to be sized the same so that things are not jumping around during the image transitions. Campus Suite will in fact create 12 different sized images based on the size of your original, but none of them may be the size needed for the banner area of your home page.
Theme-related areas, like logo and slider, you can't use the auto-sized sizes because of either they a) aren't the right size for the area, or b) the image is a PNG and those pre-sized ones made via the system come out as JPG.
If you're just dropping images into the content area of your site, say 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.