In your Sandbox you have an example called "Imagery Layers". In it you have the following
url : '../images/Cesium_Logo_overlay.png',
rectangle : Cesium.Rectangle.fromDegrees(-75.0, 28.0, -67.0, 29.75)
Cesium seems to do some blending around the edges. Is there any way to not have the blending such the the image, in this case the Cesium logo, is just rendered on top of the existing layer?