[O] Blank equation when exported to HTML using dvipng

[Richard Stanton]

Here's a very simple ORG file:

--------

#+OPTIONS: LaTeX:dvipng

* Test file
\begin{equation}
x=\sqrt{b}
\end{equation}   

----------

When I export this to HTML (Org-mode version 7.8.11 (release_7.8.11-12-gc55405 
@ c:/emacs/site-lisp/org-mode/lisp/)), the equation comes out blank. 

The HTML file contains the following code:

<img src="ltxpng/test_660c6efb146974e9d77e2de45f258385867ef389.png" 
alt="\begin{equation}
x=\sqrt{b}
\end{equation}"/>

However, directory ltxpng is empty.

I'm running Emacs 24.0.97 under Windows Vista with MikTeX 2.09 installed and 
PATH set so that I can call pdflatex, dvipng, etc. from the command line just 
fine.

Any thoughts about what might be going wrong?