Moving this issue from https://github.com/CesiumGS/cesium-ion-community/issues/276.
To visualize your imagery in Cesium ion, you need to upload the source raster (such as GeoTIFF) and Cesium ion will reproject and tile into WMTS/TMS tiles. A few people have requested the ability to upload imagery that’s already been tiled.
If this is something you’d like to see, please comment or 
this topic! For loading TMS/WMTS imagery into your CesiumJS app, see: