PG is, to say the least, an exasperating disease, both from the standpoint of the patient as well as the treating provider. It can be refractory to every treatment in Shelley and Shelley, or it can heal on its own, or it can respond one time to one regimen, then be refractory the next go-round.
PG is notorious for, in its smaller iterations, being confused with spider bites. If you EVER hear of anyone who says they get multiple spider bites with subsequent necrosis, nine times out of ten, you are dealing with PG.
The hallmarks of PG are all there for you in the pic above: Purplish, undermined skin, evidence of precious ulceration, and the history of pathergy, that is a pathologic response to trauma. Think of it like the Koebner Phenomenon of psoriasis; it is an abnormal physiologic response to external trauma.
Necrobiosis Lipoidica is a different animal in that it tends to have much smoother plaques that are more uniform, and tend to be more pretibial, although that is not always the case.
If you answered correctly, then click on this link and enjoy my favorite song from the eighties : http://www.youtube.com/watch?v=EHJmPcILfg8
If, however, you didn't, punish yourself by listening to this link, which is possibly the worst song ever recorded : http://www.youtube.com/watch?v=-rfD8NQLO8w