Some users copy paste images into the HTML editor which depending on operating system and browser results in a data uri. A data uri starts looks like this:
The image is base64 encoded and added directly to the image tag.
While it is questionable wether it is good style to use such inline data uris, it is expected that the inline data uri do also work when using the the print feature in the IMS CP viewer. Currently image data uris are not supported there.