This first part is going to be tough advice because of your lifting location - but once the weight goes past your knees on the way down, you should really drop it (keep your hands around it but provide minimal resistance and let it crash). Slow eccentrics will hurt your knees at heavier weights. Don't bounce it though - reset yourself between reps after it crashes, like you've been doing.
Pushing your hips up before the lift starts isn't a big deal, as long as you are still initiating the lift with your quads. Some people set themselves up this way, but make sure that no matter what you do pre-lift, your quads are what moves the bar off the ground and not your low back.
Your lower back looks slightly rounded to me. To practice arching lower back (and sticking your hips back), place your hands on the top of your thighs from a standing position, and run them down your legs until they hit your knees without bending over at all. Just stick your hips back and lower yourself that way.
Once the bar passes your knees, you need to force you hips forward with your glutes/ass. It looks like you are doing the top half of the lift by pulling with your lower back, instead of by pulling your hips through. When the bar gets to your knees, squeeze your hams and glutes with all you got. Your ass isn't sticking far enough back at the point where the bar hits your knees.
You have a good start. Hopefully these pointers help you.