You can delete OS images that have been imported through the Operating Systems screen, but you cannot delete the OS images that come with AppDNA (these are called system images).
If there is only one OS image for an OS family that is relevant for the reports included with your license, do not delete it.
If the OS image was set as a default OS image for a report in OS image configuration settings, AppDNA automatically resets that default. If images for the relevant OS family have been imported, AppDNA sets the most recent one as the default. If there are no imported images for the OS family, AppDNA uses the system image. If necessary, change this in Edit > Settings > OS Image Configuration.