App on MAS that writes mathematical formulae into an image? I remember an app that was featured on the Mac App Store a while ago that allows you to write mathematical formulae into an image.

I can't find it now though. Does anyone know what it is?
It isn't in the Mac App Store, but try LaTeXiT.  

As you might have guessed, it uses Latex input to create an image of the equation.  You can link the image into a Keynote or Pages document (or others, I'd imagine).  Live updating, etc.

I use it frequently, and love it!