"Mostramos" is 1º person plural, which means that the subject is "we". It doesn't make sense to say "mi amiga mostramos", because the subject is "she". The verb has to be in 3º person singular: "muestra".
Generally, the pronouns in Spanish are placed between the subject and the verb. This may vary depending on the tense and other factors, but the general rule is that. Besides, first goes the indirect object and then the direct object. Therefore, "mi amiga nos (a nosotros) las (las fotos) muestra".
Other examples:
Marta os entregó un paquete a vosotros.
Marta os lo entregó.
Mi padre ha regalado una rosa a mi madre.
Mi padre se la ha regalado.
El profesor me ha corregido el examen.
El profesor me lo ha corregido.
If you have any doubts, feel free to ask! :)