Additional meeting and call-related features and issues within Microsoft Teams for business
External users don't have permission to view profiles, so no profile photos. External users only have access to documents, data, and lists. This is a security limitation within Teams.
If you are federated with another organization, the profile photos should show.
You can vote for and comment on this suggestion https://feedbackportal.microsoft.com/feedback/idea/19e52b46-5181-ed11-a81b-6045bdaf6a9e
-- Diane