Thanks for posting! What technology did you use to create your Fire TV app? If it's something you built and are uploading to Appstore, then please review this documentation about how to submit screenshots for your Fire TV app: https://developer.amazon.com/docs/app-submission/asset-guidelines.html
If, however you used Amazon Creator to build an app for you, then please note that since Amazon Creator was only recently released, it has only a minimum set of submission related options. I have forwarded your message to the Amazon Creator team to review.
If you require custom screenshots, and potentially additional control over your app in the future, then I would strongly suggest to build your app with either Fire App Builder, Web App Starter Kit, or directly with Android SDK (an activity + WebView). Thanks!