Default image mode. Selects how the default image is applied when images specified in the request are not found.
Enum. ‘0’ to replace the entire composite image, even if the missing image is just one of several layers; ‘1’ to replace each missing layer source image with the default image and return the composite as usual.
Image Serving reverts back to
DefaultImageMode=0 when nested Image Rendering, FXG, or
req=set requests fail.
default::DefaultImage if not defined or if empty.