If the right side is your cold side, then your lamp should be on the left. The lamp will warm the hot side first and then the heat will disperse to the cool side, warming it up to the 80-84 degree range while keeping the hot side closer to 90.
You should also either buy a name brand heat lamp from the pet store or do what I did and go to lowe's or home depot and buy a 8.5" clamp lamp fixture that has a metal "shade" around the socket instead of that socket you currently have. The one you currently have is probably losing alot of heat to the air around it because of that open air design on the fixture.
I just noticed you've been asking us about heating the tank past 78 or so but you're sitting at 90 degrees hot side according to your thermometer. If you're trying to raise the cool side temp, just move the lamp closer to the middle a bit but you're close at 78 for the cool side.