Well, it looks like he is using WHOLE stamps, and I would guess it would be hard to find the exact stamp to fit in place without somehow painting over it, but I am just guessing.
There is another artist that actually cuts his postage stamps into small bits, and creates his artwork using those bits. His art seems to be a commentary and/or political satire, as he has added opinions to his work. Check out sirstampsalot.com to view some of his work.

