BuddyPress.org

get_post_meta( bp_get_activity_secondary_item_id(), ‘thumbnail’, true)

but have a read about this – if you have pull info from a user’s blog post. so to switch, you’ll need to pass in bp_get_activity_item_id() (which is the blog_id)
https://codex.wordpress.org/WPMU_Functions/switch_to_blog