it seems that the whole interface for disk snapshots works correctly only for persistent images. Though snapshots are created for non-persistent images, they are not shown in Storage->Images->$DISKID->Snapshots and they cannot be deleted through the OpenNebula Interface after the corresponding VM has been terminated. With persistent images, the snapshot is shown in Storage->Images->$DISKID->Snapshots and can be deleted there after VM termination.
Additionally, I discovered that CDROM images are never persistent. They can be set to persistent and then, when they are in use by a VM, their state is shown as USED_PERS, but in the VM->Storage information, CDROM images are always shown as not persistent.
I think it is a good idea to disable snapshot creation for non-persistent images.