Also, Pope Pius XII banned Sr. Faustina's image "in the forms proposed by" her (which were?). See this.
See AAS 51 [1959], p. 271:"AAS 51:271" said:
NOTIFICAZIONE
Si rende noto che la Suprema Sacra Congregazione dei Sant'Offizio, prese in esame le asserite visioni e rivelazioni di Suor Faustina Kowalska, dell'Istituto di Nostra Signora della Misericordia, defunta nel 1938 presso Cracovia, ha stabilito quanto segue:
1. doversi proibire la diffusione delle immagini e degli scritti che presentano la devozione della Divina Misericordia nelle forme proposte dalla medesima Suor Faustina;
2. essere demandato alla prudenza dei Vescovi il compito di rimuovere le predette immagini, che eventualmente fossero già esposte al culto. Dal Palazzo del S. Offizio, 6 marzo 1959.
Ugo O'Flaherty, Notare
"translation" said:
A notification of the Holy Office:
The Supreme Sacred Congregation of the Holy Office, having examined the alleged visions and revelations of Sister Faustina Kowalska of the Institute of Our Lady of Mercy, who died in 1933 near Cracow, has decreed as follows:
1. The distribution of pictures and writings which present the devotion to the Divine Mercy in the forms proposed by this Sister Faustina, should be forbidden;
2. It is left to the prudent discretion of the Bishops to remove such pictures which may have been already exposed for worship.
From the Holy Office, 6 March, 1959.
Ugo O'Flaherty, Notary
The Pope is certainly free to regulate the devotions which are practiced in the Church. As I recall everyone who was involved with these devotions obeyed Pope Pius XII.