Added lightbox.

Author: csmulligan

articles/active-directory/external-identities/what-is-b2b.md

@@ -84,7 +84,7 @@ You can delegate guest user management to application owners so that they can ad
 - Administrators set up self-service app and group management.
 - Non-administrators use their [Access Panel](https://myapps.microsoft.com) to add guest users to applications or groups.
 
-:::image type="content" source="media/what-is-b2b/access-panel-manage-app.png" alt-text="Screenshot showing the Access panel for a guest user.":::
+:::image type="content" source="media/what-is-b2b/access-panel-manage-app.png" alt-text="Screenshot showing the Access panel for a guest user."  lightbox="media/what-is-b2b/access-panel-manage-app.png":::
 
 ## Customize the onboarding experience for B2B guest users