Nothing in those pictures looks so bad that I would expect the drivetrain to be seized up. I would thing the chain and wheel would turn with amybe a few tight spots int he chain, but it would be rideable.
The description of the problem that it seems to pedal forward with the rear wheel off the ground but not when a person sitting on it makes me think it might be a broken axle or frame - the weight of the rider or force in the chain that pulls the broken part out of alignment and the wheel jams, either the rim or tire against the frame, or the hub against he axle where it should be riding on the bearings.