You can easily edit the .bmp files with a graphics editor.
Experiment with them a bit. You might want to make a backup copy
in a folder somewhere before you start.
As for the sky it may or may not yield the results you want because the sky is tiled, so your modification of 1 point may show up in 6 places in the sky. Don't let that discourage you, there may be uses for that also in some cases, but you won't know unless you try.
You might want to place some object in the sky instead, or an object on the ground aligned with the runway. Just a different idea that might do the same thing.
Oh and if you think flying under the bridge is a challenge, you should try flying through the windows.