[1] Apple News Format Reference: Metadata () <https://developer.apple.com/library/ios/documentation/General/Conceptual/Apple_News_Format_Ref/Metadata.html#//apple_ref/doc/uid/TP40015408-CH3-SW1>
"thumbnailURL": "bundle://header.png",
[2] Apple News Format Reference: Properties () <https://developer.apple.com/library/ios/documentation/General/Conceptual/Apple_News_Format_Ref/Properties.html#//apple_ref/doc/uid/TP40015408-CH2-SW1>
{
"role": "photo",
"URL": "bundle://image.jpg"
}
[3] Apple News Format Reference: Audio () <https://developer.apple.com/library/ios/documentation/General/Conceptual/Apple_News_Format_Ref/Audio.html#//apple_ref/doc/uid/TP40015408-CH6-SW1>
A percent-encoded URL to an image that represents the audio file, such as a cover image. Supported image types include JPEG (.jpg or .jpeg), PNG, and GIF.
Image URLs can begin with http://, https://, or bundle://. If the image URL begins with bundle://, the referenced image file must be in the same directory as the document.