[abenedik]

If you would like to show image with 3D box than you should use ImageBrush for DiffuseMaterial (note that you should add a transformation that will vertically flip the image):

<DiffuseMaterial >
<DiffuseMaterial.Brush>
<ImageBrush ImageSource="C:\mypng.png" Transform="1,0,0,-1,0,1"/>
</DiffuseMaterial.Brush>
</DiffuseMaterial>