Is it permissible for a young, unmarried woman to post her pictures on Facebook from when she was little, knowing that her facial features have not changed much?

A young girl is either at an age where she is desired or at an age where she is not desired.

In the first case: It is impermissible to publish her pictures due to the potential for fitnah (temptation/strife). Sheikh Ibn Uthaymeen said: "If a girl reaches an age where men's souls and desires become attached to her, then she must observe hijab." Publishing pictures, in this context, aids in unlawful looking.

In the second case: There is no harm in looking at her, due to the absence of desire and the security from fitnah. Ibn Qudamah said: "As for a young girl who is not suitable for marriage, there is no harm in looking at her."

It is advised not to publish personal pictures at all, due to the potential for fitnah.