Oh, the colours are now right, aren't they?
Thought that was still a problem.
I actually did not find the stretching to be a problem.
cool3a2 says the image should have a 4/3 ration.
Means you take the current width. You divide it by 4, multiply it by 3, and that result should be the height.
For a flag with the width of 128, this means 128/4 = 32, 32*3 = 96 height.
Since you cannot just cut parts of, it means you have to shrink the flag to a height of 96 pixels, and add white space above and below it til it has a height of 128.