The imacs don't have LED displays, just standard cold cathode backlit LCD.
Also, I REALLY recommend that you DON'T try fixing a dead pixel on an aluminium imac by massaging it. Considering that there's a glass panel infront of the actual screen, you'd have to 'massage' the glass so hard that it broke before you even applied any pressure to the real screen! Use one of the many 'flashing colour' videos - you have to leave them running for many many hours, but they do work sometimes (I fixed one on an old lcd that way).