Paint.net is an easy to use free alternative to some commercial image editing programs but is not free as in
you can do whatever you like. Paint.net requires Microsoft's .Net virus.
Download Paint.net from www.getpaint.net, 4.8 megabyte download without documentation or .Net. You have to download the documentation as a separate Microsoft Windows format help file and you have to download a recent version of Microsoft .Net that works only on Windows XP, Vista, and 7. You are not free to use Windows 2000, Linux, or a Windows with an old version of .Net.
Microsoft .Net is Microsoft's version of Java with a whole bunch of other stuff randomly thrown into the same gooey mix. You do not know what you get when you install .Net. .Net updates lock you into random updates that reset your file associations to use Microsoft products instead of the ones you choose. .Net updates change a random selection of language settings back to American. .Net updates are used to covertly force you onto the most recent release of Internet Explorer. .Net is used as a trojan horse to force onto your computer whatever Microsoft software is in fashion at Microsoft, whatever products have the political clout to make Microsoft executives think it is more important than your choice. In short, .Net is a virus that installs other viruses.
Time to learn
If you have not rejected Paint.net because it requires .net, consider the double training required. You spend time learning Paint.net but cannot use Paint.net on every computer or for every task, which means you have to learn something else, probably Gimp, so why would you bother learning two products for the same task if both do the same things. Learn Gimp first then skip Paint.net.
Some people compare Paint.net to Adobe Photoshop but it is not Photoshop. If you are a full time professional image editor, you will use Photoshop because your customers will request specific effects and you will have to download specific Photoshop plug in tools to achieve the effects. Gimp works with many Photoshop plug ins but paint.net does not. Gimp might be a replacement for Photoshop but Paint.net is not.
Professional photographers often use Photoshop because that can follow the many Photoshop tutorials. Some of those tutorials have parallel versions for Gimp or mention the Gimp equivalents. There is also a huge range of Gimp specific tutorials. You do not find Paint.net tutorials when you search for professional effects, leaving Paint.net way behind Gimp as an alternative to Photoshop.
Paint.net does have a few special effects and special effect plug ins that are similar to Gimp and Photoshop effects but I cannot cannot compare them because the paint.net developers will not give people the freedom to use any version of Windows and I am not willing to buy another computer just to run paint.net.
The few examples I see of Paint.net special effects are all in the ugly distortion category. These are the effects that teenagers use when they first edit photographs of their friends. Some people think the results are funny for a few seconds then become bored. If you are thinking about twisting your friend's face, or your pet's face, in a photograph, remember that 99.8 percent of people have already seen many more twisted faces that they want to see.
Paint.net is not worth the download for the few extra special effects built into Paint.net. If you need one of those special effects, look for the Gimp equivalent add on.
ACDSee, from acdsee.com, is a good low cost alternative to Photoshop for people who handle a lot of photographs and do not need to perform complex operations on every photograph. Paint.net does not have any bulk handling features and Gimp has only some of the ACDSee bulk image handling features available through crude scripts.
When you have only three photographs from your holidays and have the whole weekend to edit them, you can use any image editor. When you produce hundreds of images per day and need to quickly rotate, adjust, and crop all the images, you want ACDSee. The ACDSee Photo Editor is not the good bit, it is the ACDSee Photo Manager. Paint.net does not have the image management features you need when you have a lot of images.
Gimp can batch process images using scripts and is starting to get image management features added on. Photoshop already has some built in and works nicely with add on products. You could use the ACDSee Photo Manager with Paint.net but then you would have the the ACDSee Photo Editor and not need Paint.net.
There are no obvious reasons to choose Paint.net and several reasons to choose something else. Look at Gimp first then ACDSee for the professional image management and Photoshop for the full time professional who has to work with other Photoshop users.