Hi Felix ! I have already posted this info in another thread but if you can find it, have a look at the Nikola and Milena book, it is full of pictures of Militsa, Stana and their siblings as children as well as adults.
Lisa, I believe the fourth picture you posted is actually not Militsa but her sister-in-law, Charlotte-Auguste of Mecklemburg-Strelitz, wife of her brother Danilo, Kronprinz of Montenegro. She took the name of Militsa-Jutta on conversion to orthodoxy which makes it quite confusing ... but if you take a close look at the signature, it is "Militza, Erbprinzessin von Montenegro" and not simply "prinzessin".