When image is added with size Size.Empty, it is exported to HTML with the following invalid syntax:
width="-Infinity" height="-Infinity"

Exporting to Docx throws exception. 

Consider exposing an API for suppressing the export of 'width' and 'height' attributes to HTML, for example in the ImageExportingEventArgs.

The fix is available in our latest LIB release (v. 2015.1.0316)