Icon and minor manual update

Due to a report from a user that the executable was flagged up by some security tools, I read a bit more about it and realized that this might have been due to the lack of an application icon. So I've added one, and the tool is not flagged up anymore!

I've also updated the manual to communicate that if in the Pixelate tab you set the target width to 0, then image will not be rescaled, so you can just reduce colors and dither.