We found the cause of the breakage. The files are being generated by the Linux Screenshot tool and, by default, it names the files according to the date and time they were taken (e.g. "Screenshot from 2013-01-08 16:41:56.png"). It seems that the special characters in it (Not sure if it's the " ", the "-" or the ":") are acceptable to the Jive editor (It lets you upload and work with them) but aren't acceptable to the renderer, causing the published document to fail to render the graphic both in HTML and PDF. Renaming the files to "1.png", "2.png", etc, before upload fixes the problem.
How do I file a bug please? Either the renderer should work with these files or the editor shouldn't allow them to be uploaded.