If the original image is missing or the original image url is not accessible, you can define what default image to show to the end user who is trying to access the url.
You can also select the type of response:
HTTP 200 with default image
HTTP 200 with transparent 1x1 px GIF (V7 API only)
HTTP 404 with default image
HTTP 404 with empty image (body = 0 bytes)
