The image pricing includes the fetching of master images, processing of all image alterations, and return of derivative image. You can create multiple derivative images in different sizes, transformations, and formats without additional charges. For example, requesting 5 different sizes of a particular master image would only count as one master image being accessed.
The bandwidth pricing includes the delivery of derivative images to your users. For example, if your rendered image is 200 kB and you serve that image 100 times in a month you will be billed for 20,000 kB. Bandwidth is calculated and charged per gigabyte.