If you are using LaTeX syntax for formatting, you have to export to LaTeX or combined PS/LaTeX (PDF/LaTeX). Exporting to a plain image file like PNG does not involve any LaTeX processing. I might add that as a feature in the future, but LaTeX preview was never really meant to go into exported bitmap images. it's a feature for people who are using FIG graphics in LaTeX documents.
I would not even recommend exporting to bitmap grahics. They are not suitable for high quality printing and even less if you resize the images when you put them in a document.
I *guess* you are exporting to a bitmap in order to import the image in Word. Is that right?
If you want to add bitmap images to a Word document, better export to PSTeX and run the result through a LaTeX processor. You need some kind of minimal LaTeX file to do that. Look
here or in the
FAQ under "How can I create a LaTeX file with PSTeX or PDFTeX?".
Convert the resulting DVI file to EPS. Import that in Word. I sounds like a lot of steps, but it's not that difficult. Put all the LaTeX command line stuff in a script.
Although I would not recommend Word for a scientific publication. Better write your whole text in LaTeX. It's easier and looks a lot better. An you can still use it in 10 years without buying an MS Office subscription.