Adding to Brian's explanation,
For the transmission, I was using a 5W RS-HFIQ radio attached to an RM Italy amplifier. The radio is run by HDSDR, so I was able to pipe in audio from both FLDIGI and MMSSTV simultaneously. I continuously sent a blank PSK tone with FLDIGI and then sent various SSTV images from MMSSTV. I had Orbitron controlling the HDSDR frequency to correct for uplink doppler, so both the PSK and SSTV would stay on the correct frequency. It also took a few tries to calibrate the uplink frequency, which was about 250 Hz lower than the published value. This was important because SSTV demodulators want the tones at the correct frequencies.
For spectrum writing, a sent a message with a second instance of FLDIGI while the first instance was sending PSK.
-Stephen, N8URE