In case you’re still looking for a solution:
Go to Settings/WP LaTeX and enter “transparent” as default background color.
The template I am using applies a white background color to images, so setting the background color option to ‘transparent’ wasn’t enough. To make the formula transparent, I had to apply a little hack to wp-latex-admin.php: I changed
$css = 'img.latex { vertical-align: middle; border: none;}';
into
$css = 'img.latex { vertical-align: middle; border: none; background-color:transparent;}';
That worked for me. If someone find’s an easier way to do this, let me know. 😉
