BuddyPress.org

These two error messages indicate that you do not have a a complete install of the standard member theme.

You state that you are using the standard BuddyPress themes and it seems like you are able to access the member theme to get to the links to change the avatars, but the files to alow you to do so are missing.

For the change-avatar.php file, look in /wp-contents/bp-themes/bpmember/profile.

For the group-avatar.php file, look in /wp-contents/bp-themes/bpmember/groups/admin.

If you do not see either of the files in those locations, then your installation is flawed. Reinstall the member theme, reselect it in WPMU admin, and try again.