There is a known bug in the Nectar Dashboard in which it occasionally does not update to show when a snapshot creation has completed. Instead it gets stuck at "Image uploading" as shown in this screenshot supplied by one of our users:
In fact, the instance snapshot does successfully create, it is only the dashboard that fails to recognise the change and update the displayed status.
You can check the true status of the image using the openstack command line tools as described at https://support.ehelp.edu.au/support/solutions/articles/6000075747-api
Refreshing the tab in your browser may also help. (This will not interrupt the current state of the snapshot creation.)