Sorry for the delay, after reviewing the code base and existing uploaded files Neurovault currently does not support .mgh/.mgz files. mri_convert should be able to convert .mgz to .nii.gz.
The site at one point had been able to use pycortex from Freesurfer to visualize volumetric data onto cortical surfaces. Looks like the call to tkregister2 that generated these images is failing right now. I’m looking into getting that running again but no promises on a timeline.