Shon7,there are lots of good books at the library on big jets,and I suggest you start there.Lots of good sites on the 'Net as well. Firstly, pilots (and passengers)can feel the touchdown,(it happens at about 2 ft/sec). Double bogied gear (4 wheels)contact in two phases (usually rear followed by front). The pilot must then de-rotate to bring the nose wheel onto the runway (this can also be felt by all on board, depending upon pilot skill). Various sensors on the landing gear determine 'weight on wheels' or ground mode. Some equipment, like thrust reversers or ground spoilers, require main gear on ground. Wheel speed (rotating) can also be used on some aircraft to initiate spoilers and autobrake. What you saw in the photo, was reverser deployment, with the nose gear still in the air. This is possible, although the interlock on the reverse thrust levers would not enable the pilot to 'pull reverse thrust' (increase engine power) until the nose gear was compressed(otherwise,the nose may slam down onto the runway). Thrust reversers (not reverse thrusters)merely block the fan bypass air (the air that is produced by the fan, but does not flow through the engine core)and reverse its direction through cascade vanes. The fan produces about 70% of an engine's thrust, so by reversing this air, braking can be achieved. Low or no bypass engines use reversing buckets behind the exhaust. These were tried on high bypass engines in the early days, but the cost, weight and maintenance didn't make them feasible. Reversing the fan air, along with wheel braking and ground spoilers does the job just fine.