A Master Image is any image requested by imgix from your connected source (e.g. an Amazon S3 bucket). Derivatives of that same image (such as resizes and crops) do not count towards your Master Image count.
For example, this is a single master image:
...while all of of the below are derivatives of the image above, and are not counted as additional Master Images:
We recommend uploading the highest-resolution version images for use as Master Images.
If I purge a Master Image, will it be counted again as another Master?
We only count masters by URL paths in a single month. If an asset has already been counted for a month and it's purged and accessed again, it will only register as a single Master Image for that month.
If you removed the file from your origin and purged it so that it returns an error, it will not count as a Master Image only if there was no successful request to that image during that month. Otherwise, the count will reset next month and requests to the removed image will not register as a Master Image since it will return a
404 error instead.