Today I learned (via support):
When images are copied and pasted from a Google Doc into a CC article, they don't get uploaded to CC's storage. Instead, the article just references the original external URL where Google was hosting the image. If that link later expires or access changes on Google's end, the images break.
To avoid this in the future, I'd recommend downloading the images from the Google Doc first and then uploading them directly into the CC article using the upload tool. That way, the images are stored on CC's servers and won't be affected by any external changes.
Bolded emphasis mine.
