Paint.NET

Paint.Net can't even be compared to the famous picture editor from Microsoft, as it is a complete and compact multipurpose image editing suite with endless features that put it at the top of its category. Among its features, you'll find many that other powerful suites like Photoshop include, such as layers, history so you can undo actions, advanced filters, professional tool bars ... and yet, it's still an extremely useful tool for drawing pixel art, and can equally be used for retouching photos or even for working with digital drawings. Paint.Net supports many formats, including some of the most well-known ones like BMP, PNG, TIFF, JPG, and GIF, as well as including many post-processing tools that can be used to get professional results: blurring, red eye reduction, bevel, perspective, noise, etc. You can also edit the color properties of images by adjusting brightness, saturation, contrast ... It is, without a doubt, one of the best free tools you can find.