[Chrome] Unable to take screenshots of elements within iframe

I am trying to take screenshots of elements in iFrame using BackstopJS+Chrome but it seems like there is no way I can do it because the snapshots could not be taken by specify the selectors of those elements in the "selector" property for the backstopjs file (I can take screenshots of 'body' and 'iframe' just fine but would like to take snapshots of elements in the iFrame)

Is there anyway I can do this? Would it be possible to create a Chromy script to take a snapshot of elements in iFrame and pass them back to backstopjs (e.g. specifying a Chromy script to take snapshots in iframe and specify the script name for he 'onReadyScript' property?

Any help would be greatly appreciated!