not to be negative about those guys, but you can tell a lot about a rider's overall skill level by what they're doing in pictures. the odds of catching a lousy rider doing it correctly exactly when you happen to be taking your shot are worse than winning the lottery! point being, if they look good in pix, then they are probably doing it right most of the rest of the time.
from the pix you took, i would say none of these riders are beyond intermediate level, at least not here in texas. the exception may be the dude doing (starting to do?) the nac nac. if he went ahead and threw that one completely sideways, then he's got ALL my respect. number 72 needs to work on his technique (the "body whip" in #2 is a dead giveaway!
), and any c-rider can jump as well as the honda guy in red gear. obviously, there's no guarantee, but that's what the shots communicate to me. having said that, reality is that jumping shots are usually the worst for showing a lot of skill, unless the rider is throwing a big whip, of course! corners are where you really see it.
check out some moto mags and this may make more sense, then next time at the track, watch and see the difference in what the fastest and slowest guys are doing. there are also several shooters on here worth looking at - squid31, browndogwilson, and several others for sure.
that's my perspective from being a rider for 20 years and a shooter for half of that as well. but, at the end of the day, what's the objective? if you want shots of all riders to sell back to them, then you take what you get. but, if you're looking for a few great shots that show the beauty and grace of motocross, then you have to focus on the great riders!
hope that helps. keep shooting and posting - there are quite a few of us here that love moto shots!
wp.