There's lots of thing involved in making passable screenshots, but it all starts imho with the right addons.
The right clouds, the right scenery and the right aircraft.
In your shots, I see mainly default stuff, try to avoid that.
Even if you limit yourself to freeware, there is some good stuff out there. Try
here for scenery for example. I was quite happy with FS Skyworld for the clouds in the old FS9 days. Nice aircraft can be found all over.
But don't be afraid to try some payware every now and then, it usually (but not always) looks better.
Then it comes to taking the shot. Sometimes you can take shots while happily flying along, but quite often, I try to set up a certain shot. I have a picture in my mind and try to recreate that. Above all, do not be afraid to experiment, with angles, lighting, weather, time of day, etc.
One final thing: try cropping the image occasionally, by cutting off the non-interesting parts.
Take your 747 shot, I would have cut a bit of the runway, and possibly the sky too, ending up with this:
Good luck,
Jan Kees