Load image before show by imageMaterialProperty

CesiumJS
2

Notice that the image parameter can be a URL, which CesiumJS will then load, or an Image element:

https://cesium.com/docs/cesiumjs-ref-doc/ImageMaterialProperty.html?classFilter=ImageMaterialProperty

So you could fetch the image with the Resource class:

https://cesium.com/docs/cesiumjs-ref-doc/Resource.html?classFilter=Resou#.fetchImage

Wait for the promise to resolve, get the Image element, and pass that to the entity. Let me know if that works.