Hello Juan,
Welcome to Atlassian Community!
Thank you for sharing the screenshots.
For team members avatars to appear on the backlog or board, it's necessary to assign tickets to them.
If they are not assignee on any ticket, their picture won't appear, because that option is actually the "filter by assignee".
Regarding your second question, I can see that you are a user on that site and the site-admin is a person from the same domain as you, so probably they are also your teammate.
If you have any other questions regarding this matter, please let us know.
Regards,
Angélica
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