Update product images on plugin and samples Unreal Marketplace page

[argallegos]

The screenshots we have on the marketplace pages for the plugin and for Cesium Samples have not been updated since the initial launch. They may not reflect our best media anymore.

We should curate a selection of fresh photos that highlight the plugin and its capabilities. This is also a good opportunity to add specific screenshots of Cesium for Unreal Samples maps.

Note that the Marketplace requires 1920x1080 images. We can have up to 25 images on each page.