Albus Dumbledore & Minerva McGonagall
She had been sat in that large cylindrical room for some time now. Her pointed hat, usually standing proudly on her head, was discarded on the desk; drooping and no longer magnificent. Minerva McGonagall wouldn't - couldn't - tear her eyes from the photograph. She had been searching through his things when she came across this particular one.
Albus Dumbledore was looking up out of the frame; his eyes shining behind those half moon spectacles. Beside him a seventeen-year-old Minerva was laughing. Her long mousey hair lifting in the wind and dancing around her face. They were stood there clad in Hogwarts uniform, thick woolen scarves hanging from their necks, flecks of snow clinging to their eyelashes.
Tears were glistening in her eyes as she put the frame down on the desk.
Minerva looked up at the Albus hanging from the wall; surrounded by all of his predecessors. He smiled down at her; his eyes crinkling. "Albus, why did you leave me?" Her voice was shaky and full of suppressed emotion.
"I never did, Minerva."
She got up and with a swish of her cloak was in the doorway, " I loved you." She said, leaving. She had, for a very long time.
The room was quiet; portraits sleeping silently. Albus, looking down at the abandoned photograph, sighed desolately.
"And I loved you." He said to the empty room.
Of course this was written before Pottermore was opened to 1 million Harry Potter fans (Potterheads) and, therefore, isn't accurate. (Rowling recently released that Dumbledore was a teacher whilst McGonagall was a student...)