It appears that we depend on the image the instance was originally created from to seed the new snapshot image with certain attributes (disk_format, container_format, architecture). So at this time, it is a requirement to have the original image available in order to take a snapshot. I do think we should redesign snapshotting so we don't have to depend on the original image, but I don't think that is feasible for the essex release timeframe.
It appears that we depend on the image the instance was originally created from to seed the new snapshot image with certain attributes (disk_format, container_format, architecture). So at this time, it is a requirement to have the original image available in order to take a snapshot. I do think we should redesign snapshotting so we don't have to depend on the original image, but I don't think that is feasible for the essex release timeframe.