Hmm, OK, well, it *says* that it output PNGs, and doesn't show an error there. Are you able to find them in this directory called "QC/"?:
++ Writing one 1416x1032 image to filter '/usr/local/bin/pnmtopng -compression 9 > ../QC/t1w_qc_xlzd.sag.png'
++ Writing one 1152x1032 image to filter '/usr/local/bin/pnmtopng -compression 9 > ../QC/t1w_qc_xlzd.cor.png'
++ Writing one 1152x1416 image to filter '/usr/local/bin/pnmtopng -compression 9 > ../QC/t1w_qc_xlzd.axi.png'
and
++ Writing one 1416x1032 image to filter '/usr/local/bin/pnmtopng -compression 9 > ../QC/t1w_qc_ref_in.sag.png'
++ Writing one 1152x1032 image to filter '/usr/local/bin/pnmtopng -compression 9 > ../QC/t1w_qc_ref_in.cor.png'
++ Writing one 1152x1416 image to filter '/usr/local/bin/pnmtopng -compression 9 > ../QC/t1w_qc_ref_in.axi.png'
--pt
