paint.net is an image and photo editing software with support for layers, unlimited undo, image effects and a tabbed thumbnail interface. The program includes a variety of paint bushes and selection tools as well as image filters such as sharpening, red-eye removal, distortion, noise etc.
The program supports plugins and will automatically start in 64-bit within an operating system that supports it.
Use 4.3.12 for: Windows 7 (32/64-bit) [with Platform Update], 8.1 (32/64-bit), or 10 (32-bit). It can be downloaded from here or here.