In order to publish a Writing NFT with a custom cover image, please follow the steps below:
1. After editing your entry, click Publish to open up the publishing modal
2. Click Customize to change the NFT image
3. Choose or drag and drop an image to use as your NFT image
4. After uploading the custom cover image, click Sign and Publish
5. Once you've finished publishing, you'll see the entry published with the NFT cover image