The following is actually a bug (IMO).

You can create a CMS item through the API (or Zapier), which includes a rich-text field. Webflow allows us to publish our own HTML there, with images in it, which is great.
Though if you would edit that item after the facts from webflow, it will actually break all image URL's. It seems the editor changes the image URL domain to uploads-ssl.webflow.com without actually sideloading those images.

I see 2 possible solutions to this:
1. Sideload the images before changing the domains.

2. Don't thinker with the image domain alltogether.

Both seem to be valid approaches.
I hope I was able to clearly explain the issue.