Insérer un visage en médaillon rond dans une vidéo

Thanks for response.

I refer to following page:

https://helpx.adobe.com/premiere-elements/using/superimposing-transparency.html

and the part and exemple  Matte

Matte

A file or channel that defines the transparent areas of a clip. The matte determines the level of transparency in the resulting image. In Premiere Elements, you use mattes in conjunction with the Track Matte Key.

There is a picture with a black area and a white circle inside to insert the head of a child.

Matte (left) defines transparent areas in upper clip (center), revealing lower clip (right)

There is a picture with a black area and a white circle inside to insert the head of a child. This is exactly what i would like to do.

I didn’t find this Matte

As I understand It should be there:

Create transparency with the Track Matte Key effect 

1. 1. If you haven’t already done so, add the matte file to the project: Click Add Media and choose Files And Folders. Navigate to the matte file, and click Open

Doing that I open my explorer and than…? where is this matte file???