PNG images, with an alpha channel, can be added to a higher VP track. Then apply the transparency effect to blend the face into the mountain range that's on the lower track.
If your face is not a PNG, but has a solid background, with no trace of the background color in the face itself, green screen and transparency effects can be employed.
Click the FX button on the clip in the time line. Click the large green + sign at the left on the toolbar. Choose GREEN SCREEN. Click in the preview window and an eyedropper will appear. Move the dropper to the unwanted background area and left-click. That will eliminate the background color from the image. See this page for details.
Other effects can be added, such as scale (to size the image) and position.
