Just as notepad++ is a must-have to replace notepad, Paint.net is not only THE mandatory replacement for Paint, it's also IMHO the little "free" brother of all those well known professional editors. It's a indeed a freeware (Creative Commons License).
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I dislike to use "pro" editors as they are usually too complex for my needs, i.e.: full of over-killing features for a end-user like me. On the opposite, Paint.net offers a clear, pro but still simple interface with all the required powerful tools such the Magic Wand for selecting regions of similar color (ex.: grab clouds in order to remove them), and the Clone Stamp for copying or erasing portions of an image (ex.: rebuild a collapsed wall).
I really use Paint.net a lot to completely remove elements from photos or rebuild their background as well as to simply make pictures' background transparent...
I like also how simple I can replace a color with another one in a picture using the Recolor Tool.
Tip: I was recently trying several free tools to determine the code of colors seen in web pages. Actually, the most stupid simple solution to get a color code is to take a screenshot (Using the "Prt Scr" key), to paste this screenshot in Paint.Net and grab Hex Codes using the Color Picker tool.
Also, to automatically edit all images with Paint.Net instead of MS Paint, I did set "C:\Program Files\Paint.NET\PaintDotNet.exe" "%1" as Default value for the registry entry: HKEY_CLASSES_ROOT > SystemFileAssociations > image > shell > edit > command. Find attached to this post the reg file that can be used to update the registry.