\Preamble{html}
\begin{document}
% Upper case PNG file extensions
\Configure{graphics*}
{PNG}
{\Picture[pict]{\csname Gin@base\endcsname.PNG align=center border=0}}
% Lower case png extensions
\Configure{graphics*}
{png}
{\Picture[pict]{\csname Gin@base\endcsname.png align=center border=0}}
% Problems with spanish babel
\makeatletter
\let\ifes@LaTeXe\iftrue
\makeatother
\EndPreamble