Muatassim, 37 — a senior army officer — appears to be in a hospital after being captured by rebels.
However soon afterwards he is seen lying dead on a stretcher, shot through the neck and abdomen. The pictures raise questions over how Muatassim died after being captured alive.
Another of Mad Dog's sons, heir apparent Saif al-Islam, 39, was last night believed still to be on the run.
He is wanted by the International Criminal Court for crimes against humanity.
Two more sons — Saif al-Arab and Khamis — have been killed in the conflict. Saif, 29, died in a Nato bombing raid on Tripoli while Khamis, 28, perished in fighting near the capital in August.
Mad Dog's remaining four children appear to be safe after fleeing Libya.
Ex-footballer Saadi, 38, went to Niger where he is still in hiding.
Sons Hannibal, 36, and Mohammed, 41, and daughter Aisha, 34, are all in Algeria after they were given refuge by the government there alongside Gaddafi's wife Safia.
