Hort,
That's not exactly even a good snapshot.
Look at the speed, G load, velocity vector of the F-18 (caged low at the bottom of the hud), and the fact that the gun cross and F-18 are not in plane with the F-22. It looks a bit like a no-lock high angle snapshot, with the F-18 pretty much falling out of the sky.
Low PK at best. Yea it's a cool picture and the fact that the F-22 was even threatened at all is a bad thing for the F-22 pilot (heh) but I wouldn't expect the F-22 to be brought down by that shot.
The F-18 can move the nose fairly well, but right there it's over 20 deg nose low (with the velocity vector even lower) so there isn't any way the F-18 can follow the F-22 through that maneuver.
I found a discussion about that pic from back in March... Basically the discussion over there was pretty much the same - it's probably a valid low-PK snapshot. Regarding the comment on how the ranging could be valid even though there is no radar lock... er... I can't talk about radar modes except to say that the displayed range could be correct or incorrect depending on a lot of factors.
http://www.worldaffairsboard.com/military-aviation/37371-raptors-debut-red-flag-wield-unfair-advantage-3.htmlApparently there are more pics over there too.
As for the "impossibility" of the shot, I don't think anyone ever said that it was impossible, merely unlikely in training and even less likely in a "fair" fight (meaning both sides are allowed unrestricted real-world tactics).