A number of people have reported problems after upgrading to version 4.3.0 or 184.108.40.206 of Domain Mapping. In my case, and a number of others I’ve read about, the plugin started replacing the domain of my subsites with the domain for my network site in URLs and permalinks. So mappedomain.com/files/image.jpg became networksite.com/files/image.jpg. As a result, all the links to images on all my subsites stopped working and no pictures were being displayed on any of the sites.
I assume the WPMU Dev team is working on a fix. In the meantime, I was able to get my subsites working again by deleting the Domain Mapping plugin folder (which had been updated to 220.127.116.11) and uploading the folder for version 18.104.22.168 in its place.
The older version of the plugin is available on the Domain Mapping changelog page or via this direct link to the version 22.214.171.124 download.