Images that are used as overlays, watermarks, etc. will count as master images even if they are not addressed or served directly. For example, if you have 5 images of cars and 1 image of a logo, and you use the logo as a watermark on each of the car photos, you would be billed for 6 master images. The logo image may never be delivered directly, but still counts as an master image since it is fetched to render the logo watermark.
