Still Probably The Best Free Photo Editor For Windows [1]

And the most recent version, which is now 4.0.6, is looking as good as ever.
If you don't already have a good photo editor installed, Paint.Net is definitely worth trying. And if you have an older version on your PC already, be sure to upgrade in order to get the most recently added features.
You'll find the download at http://www.getpaint.net/download.html#download [3] and it runs to around 6.5 MB. The program is malware-free according to VirusTotal and Web of Trust, and runs on Windows 7 onwards. It worked flawlessly under Windows 10 on my test machine.
The program is free (though there's the opportunity to make a donation to the author if you wish).