I found that problem happens when there's no \begin{document} or
\startdocument command. And with the last, but if \documentclass{}
(possibly hidden in {htmlonly} environment) is missing, another error
occurs; log looks like
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pstoimg: Error: "/usr/bin/pnmcrop < /tmp/l2h23297/p23315.pnm | /usr/bin/ppmquant
-floyd 256 | /usr/bin/pnmtopng -interlace -trans '#ffffff' > img1.png" failed:
Illegal seek
Error while converting image
----
But this is because of missing \documentclass in images.tex. Without
\startdocument, some defaults are placed there (but not your packages)
Vadim
