I have mailed their support two times in a row, but there was no response.
closed as off-topic by Al E., Vidar S. Ramdal, codingbadger♦ Oct 1 '15 at 11:03
This question appears to be off-topic. The users who voted to close gave this specific reason:
If you are willing to migrate away from Posterous, then I'd suggest you add MathJax to the blog to do this; it's what's used on sites like math.stackexchange.com to render mathematics and works really well.
If you're not, then I think you'll need to do some pre-processing of your Posterous posts to detect LateX, render it, save it as an image, and replace the LaTeX with that image in the post. Not really a nice way to do it, but it seems there's no other way.