I use two methods.
The main I use is a programme called Kolourpaint. I have one computer with Ubuntu as its operating system which supports Kolourpaint, Windows does not.
The other programme I use is Gimp, which is supported by Windows. Both are free.
I prefer Kolourpaint as it does not alter the colours and there is little pixel adjustment to do. Gimp distorts the colours and you end up with a lot of grey fuzzyness around the image.
A trick with Gimp to prevent major colour changes is to rotate the image but not click on the rotate button but to press the print screen button on your key board. Then open paint, paste the screen image to paint and clean up the rotated weapon. The colours are saved but the image is not s sharp as with Kolourpaint. I normally only use Gimp when I cannot be bothered to boot up the other machine.