The dynamic container background image selection requires a random image to be already uploaded to the container as a background for the dynamic functionality to act. Are there plans to improve on that? Uploading a random image first seems like a hack.
personally i like the current method – some reasons:
1. Auto added images are not reliable – for example if we used the latest post featured image, what happens if the post has no featured image.
2. This method provides the fallback image option – theres no extra step to add one – just a simple toggle.
3. You can choose and change the background image when designing the element which i rather like.
4. Templates – as the background image can accept an external URL you can save a distribute HTML templates with an placeholder image.