Hi Laura,

I'm the developer for SMILI, http://www.nitrc.org/projects/smili,
which is a cross-platform open source viewer with modern integration with a number of Linux distributions including OpenSUSE, Ubuntu and Fedora including packages. Windows and Mac OSX versions are also available.

You can convert your Nifti's to PNG via a simple save button after opening it to capture the data exactly or use the screenshot feature to grab a view of a slice. Here a quick demo of various features
http://smili-project.sourceforge.net/get...
and some tutorial videos on the YouTube channel
https://www.youtube.com/channel/UCD-hU6I...

Is there a reason you wish to save as PNG's? My understanding is that the PNG is 2D only and limited to 32-bit integers.

Let me know if you encounter and issues or bugs or even if you have suggestions.

Cheers
Shakes