All it proves, like others already suggested that there were/are BUK SAMs in Ukraine and is of the same significance as discovering that there are Ford Trucks in the US.
Clearly not all the pictures are of the same one, either. Open, covered, flatbed, self-propelled -- not the same one.
But there is one with a missile missing. How many others have they been using, lately? that we know of? did someone casually video one with a missing missile weeks ago, then post it a day after the plane went down? was Luhansk bristling with Government troops by then? or just been recaptured and left alone? would anyone non-military go up and challenge a passing missile launcher as to who's driving it ? especially if they're in uniform? or even anyone military if not at a checkpoint?
OKAY let's say it was an official one, not a separatist one. How does that change the big picture?
* the pressure is off Russia
* the pressure is on Kiev big time
* very red faces for USA
* lot less excuse for official troops to make this mistake, as assumedly they'd have more command and control in place to identify it correctly
I absolutely can't see it being deliberate, ie a shot at a known civilian craft.