Trying to do a "clipping mask" a video to an image in After Effects
Track mattes work by applying the alpha (transparency) or luma (≈ brightness) of one layer to the transparency of another layer. So in the case of an alpha matte, if the upper layer is transparent in an area (or opaque for alpha inverted), the lower layer will be too. In the case of luma mattes if the upper layer is black (or white for inverted*) then the lower layer will be transparent. The amount of transparency is controlled by the Luma of the matte layer. 

In your example To get fully transparent / opaque mattes use curves or levels or brightness and contrast effects to adjust the contrast of the upper layer. Or use whatever you used to create the image and output one with a transparent / black background.

 *could be the other way around, I always forget