Is your goal to display imagery on top of 3D Tiles? If so, this isn’t currently supported in CesiumJS, but we have a feature request for that here: https://github.com/CesiumGS/cesium/issues/7591
You can drape your imagery over your 3D terrain if you load it as a terrain provider in CesiumJS. What source format is your terrain in? If it’s a GeoTIFF you can upload that to Cesium ion (https://cesium.com/ion).