I think Charl was correct. As soon as you start 'importing' bits and pieces it is no longer a screenshot, it is FS art.
By all means, fiddle with the colour/contrast/lighting, add some 'effects' like blur/engine exhaust haze etc... but that should be the only sort of editing allowed.
UNLESS, it is specifically labelled as a 'no holds barred' competition... then people are free to cut and paste whatever they like into the shots.
Personally, I very rarely vote for pics that have 'real world' bits cut into them, simply because I dont consider them screenshots. I'm looking more for originality, composition and how well it fits the theme. Some of the ones I have voted for have had mean as jaggies, or 'average' lighting, but I thought the idea behind the shot was great and deserved recognition.
As for those with the best PC being able to get the best shot, I disagree... there is no reason you need 20+FPS to be able to capture a good pic. All you need to do is line up a good shot (slew, rotate etc), pause, save the flight, then crank everything up to MAX and reload the scene. Sure your PC might suffer a little and you'll have to wait, but it will render it eventually... then printscreen and you're done!
But whatever the final decision on this, as long as it is clearly spelled out in the rules what is and is not acceptable, then there can be no arguments...