Dear Francis,
To round off this issue, the author of dvipng has agreed to revise it for the next release, presumably 1.18. The problem, if I understand it correctly, is that the bookmark dvips driver, which is used by default, inserts what is essentially an empty image, which dvipng does not expect, and when trying to compute the resolution it divides by zero. The bookmark dvips driver works as it does to avoid a clash with another package; I think that was the recent change that upsets dvipng.
That's great! We can hope that they manage to pack the fixed version to TL 21.
Best regards, Michal
