webmapserviceimageryprovider transparency issues

Sir I want to know why is it that when I am using the default image/JPEG format for my geoserver wms layer my layer got displayed on the cesium globe, however when I passed on the paramters like format image/png and transparent to true options nothing got displayed. I will be very grateful if you can provide me hands on for solving it.
I am working offline with NaturalEarthII as base layer.

Rosly Lyngdoh