Is it permissible for a girl to send her picture without a head covering to a young man whom she considers her “brother” online, and what should they do if this action is forbidden?

A woman's forbidden sister is her sister by lineage or by fosterage. Considering a girl who is a stranger to be her brother has no legitimate religious basis and does not establish a prohibited degree of relationship (mahramiyyah). The relationship between two strangers outside the framework of a valid marriage is a sinful relationship. Sending pictures and speaking with a stranger is not permissible except out of necessity and with legitimate religious controls. They must repent and distance themselves from this relationship.