You can load STC files into mne_analyze and visualise them interactively (File -> Manage Overlays -> Load stc). You also need to load the brain surface used to compute the source estimates (e.g. the average across subjects, if this was used to morph the single subject results).
You can also produce JPG files or movies using mne_make_movie. Here is an example on how to do it:
mne_make_movie \
--subject average \
--smooth 5 \
--stcin infile.stc \
--tmin 0 \
--tmax 400 \
--tstep 10 \
--pickrange \
--noscalebar \
--nocomments \
--jpg outfile.jpg \
--fthresh 0 --fmid 0.0666 --fmax 0.1