Hi Jason,
As part of our API response cleanup, we have decided to remove the image_url from the response because the internal implementation of our image storage has change, and we cannot guarantee that the image_url which was given as part of the API response will remain static.
If we want to see the preview of the creative, the best practice is to preview your creatives from the UI, because depending on your creative types, we will enrich those creatives with an additional script like mraid for Javascript creatives, and so on.
Thank you.