How to make an png image become white silhouette ?

I may not have understood your question correctly.
Can't I just draw a white silhouette and write it out in PNG format?
If you can, please give us your results and an ideal picture.

AI is the wrong tool for editing pixel based images. That's done in Photoshop, where it might be as simple as filling the area, assuming the PNG already has suitable transparency. Otherwise you would have to trace the outlines in AI and fill them.

Mylenium