I posted an update for a different bug but I think they might be related after some further investigation.
Basically, when this setting in the domain mapping plug-in is unchecked
"Redirect administration pages to site's original domain (remote login disabled if this redirect is disabled)"
all uploads get an HTTPS URL. If every domain map site on the network install does not have a valid certificate then images do not load. However, if that checkbox is checked this does not seem to happen in the URLs are created as http and they use the original network URL.
Of course if it is checked there are cookie and redirect issues in the current version. Basically, you get redirected so many times to different login pages even when you are already logged in. Is it possible that these are related? Any chance this has been addressed in a development version as well?