Paint.NET is not available for KDE Plasma but there are a few alternatives with similar functionality. The best open source KDE Plasma alternative is Krita. If that doesn't suit you, our users have ranked more than 100 alternatives to Paint.NET and three of them are open source and available for KDE Plasma so hopefully you can find a suitable replacement. Other interesting open source KDE Plasma alternatives to Paint.NET are KolourPaint and showFoto.
Free, open-source painting software designed by artists for artists. Offers customizable UI, 9 brush engines, color management, tablet support, non-destructive editing, and privacy features. Portable and available on major platforms, supporting seamless texture creation.
KolourPaint is a simple painting program to quickly create raster images. It is useful as a touch-up tool and for simple image editing tasks.
You can use it to view your photographs and improve them. Showfoto is the standalone image editor of the digiKam project.
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There isn't anything else exactly like paint.net, but Krita comes the closest in terms of features and ease of use. It's relatively intuitive, has various customization options, and is sufficient for basic photo editing and more advanced image manipulation and drawing.