Currently, there is no option to upload a custom virtual background and make it available to all users in the organization. However, each user could individually upload the custom background in their app. Sending around instructions to everyone on how to do this may be the only solution at the moment.
There is a feature request for this here. You can upvote it to encourage Cisco to make it a priority:
https://ciscocollabcustomer.ideas.aha.io/ideas/WXCUST-I-858
Actually, I see that they have already marked this request as "Planned". No ETA though.
