How are they achieved? Is there some script for Illustrator which alters the image adding the shapes according to the colors or shades? How would I transform a normal image of Marilyn Monroe for example, to a polygonal illustration of her?
As I mentioned in the comments, these illustrations were most likely done by hand, as the level of complexity, detail and 'originality' is not something you can achieve with a script.
You could potentially use effects like Mosaic (with triangular shapes), but the results are not nearly as good as your samples. Your best approach would be to draw each color area yourself.
However, I found this process that actually seems to give some nice results:
The Delaunay Raster is an original graphical process I invented in 2008, to create abstracted versions of images using the Delaunay Triangulation, color averaging and hand assisted placement of points.
Delaunay Raster was developed using Scriptographer and color averaging by Jurg Lehni.
However, the script is not available in the website, you need to contact the owners if you want to use it.
Some fun from Vitaliy at another stack exchange site:
This is based on Delaunay triangulation. It is far from perfect but it can help and the source code from such a sketch can be modified. Surely many other frameworks allow for such mathematical tesselations but as other answers explain finer work is often done "by hand".