A base64-encoded string of the initial image that will be scaled to 1024x576. Images should be in form of data:image/{format};base64,{base64encodedimage} if it's smaller than 200KB. Otherwise, it needs to be uploaded to a presigned S3 bucket using NVCF Asset APIs.Once uploaded you can refer to it using the following format: data:image/png;asset_id,{asset_id}.Accepted formats are jpg, png and jpeg.