If you drag an exported json file onto lottiefiles.com, it will preview it directly in the browser. You can also share the link or scan the ar code with the Android or iOS app to preview it there as well.
You can preview or take an svg snapshot of the animation to use as poster. After you render your animation, you can take a snapshot of any frame in the animation and save it to your disk. I recommend to pass the svg through an svg optimizer like https://jakearchibald.github.io/svgomg/ and play around with their settings.