Configuration of image output type (png/gif) is not straightforward because there are several places for configuration. (At least according to the php comment lines)
cookbook\latexrender\latexrender.php, line 23 is the correct place for configuration (which you may forget sometime after installation)
pub\latexrender\scripts\class.latexrender.php, line 48 has no influence despite the php comments
pub\latexrender\scripts\latex.php, line 37 has also a comment that asks for configuration
background: I needed this because my Imagemagick (7:126.96.36.199.dfsg1–2.1/debian) didn’t get antialiasing right with gifs - showing any gradually fading pixel in background colour. When using png smooth edges work just fine.
$LatexrenderImageFmt = 'png';