I had a problem with MediaWiki math sections not always displaying as a PNG. For simple expressions HTML was used instead. This lead to a very non-uniform look and feel where some images had a green background and large fonts (for PNG expressions) compared to a black background and different fonts (for HTML expressions). I wanted a uniform look and feel so I went looking for a configuration setting.
I haven’t been able to figure out how to force mediawiki to always display a PNG as a global setting, but in your user settings on the math preferences section you can change from the default “HTML if very simple or else PNG” to “Always render PNG” which fixes the problem on a per user basis, which is good enough for me.