Hi Christian,

Nilearn is very versatile. On top of Vanessa's answer on plot_roi, which is spot on, you should keep in mind that you can plot a first image controlling the fine details of things like colormap, threshold, ... using plot_img ( http://nilearn.github.io/modules/generat... ) and then capture the 'display' object returned, and add any overlay that you want, as detailed on 
http://nilearn.github.io/building_blocks...

The object returned is very rich, with a lot of methods to control the fine plotting, as detailed here:
http://nilearn.github.io/modules/generat...