Convert WPOSTX to JPEG

WPOSTX format is a text, not an image, so there is no way how you could convert it to JPG. You could of course screen capture .wpostx file opened in Windows Live Writer, but that's hardly wpostx to jpeg conversion although it basically achieves the very same thing.

Simply put, it is impossible to convert WPOSTX to JPEG directly.
Thus, there is no so-called wpostx to jpeg converter or a free online .wpostx to .jpeg converter to be found.