You have to tweak the original pictures before the conversion, so they are suitable to be converted.
One basic thing you should take care of, is that the gradients (in the original picture) should only use one color. For example:
light blue to dark blue ---> likely a good result
light purple to dark blue ---> likely a bad result
Also if the basic color is similar to a color present in the palette, this is another step for a good result.
It is impossible to describe a technique to make a picture suitable, since what you have to do is highly dependent on the original picture; and there might also be several methods to achieve the same result.
One overkill but effective technique is to turn the original picture into grayscale, then recolor it using the colors from the palette. This is easy to do in pictures like the one with the B2 bomber, since it is just a gray object on a (ideally) blue sky. Still, a certain amount of tweaking might be involved.
... But of course all of the above make sense if you know how to use photoshop (or gimp). If you don't then you're wasting your time.
In the latter case, you should just search pictures that are suitable from the start: gather a bunch of B2 pictures, convert all of them and keep the one that gives you the better result (I still do this even if I know how to use photoshop, starting from a suitable picture is always better than having to tweak it a lot).
