I'm working with some input RTF files that contain embedded PNG images. UNRTF would complete with no warnings or errors but product "empty" 0-byte PNGs. The output HTML produced in-line comments like this:

<!--picture data found, picture dimensions are 0 by 0, depth 1-->

I traced this to the input RTF only containing the following <pictsize> keywords in the {\pict} structure:

\picwgoalN
\pichgoalN

It was lacking these <pictsize> keywords:

\picwN
\pichN

I presume UNRTF is parsing the image dimensions from these two keywords, which are missing.

Since I'm not sure when this library will be updated, the work-around was duplicating the \picxgoalN keywords to \picxN keywords with the same dimensions, within the \pict structure.

See image section of the RTF spec for more detailed information:

https://www.biblioscape.com/rtf15_spec.htm

Hope this helps someone!