GIMP is described as 'Free open-source suite with advanced photo retouching, composition, batch processing, plugin extensibility, custom scripting, and wide file format compatibility' and is a leading digital painting tool in the photos & graphics category. There are more than 100 alternatives to GIMP for a variety of platforms, including Windows, Mac, Web-based, iPhone and Android apps. The best GIMP alternative is Krita, which is both free and Open Source. Other great apps like GIMP are Paint.NET, Adobe Photoshop, Affinity and Photopea.
Bazaart® is an award winning photo collage and design app. It allows you to create masterpieces on the go, and seamlessly work with Adobe Creative Cloud. Bazaart is available on iOS and Android devices.
You can use it to view your photographs and improve them. Showfoto is the standalone image editor of the digiKam project.
SketchPort is an online drawing community with free software for PC, Mac, iOS, and Android. Draw, chat, vote, be inspired, be challenged and earn badges!
Verve (Dictionary: Energy and enthusiasm in the expression of ideas, especially in artistic performance or composition...) is the result of a curious idea I had to try using the graphics hardware to run a fluid dynamics simulation.
JugiPaint is a painting software with diverse sets of tools for various types of illustration work. It provides many standard digital painting features like layers, brushes, selections, transformations, etc., as well as some less often like guide layers and panels.
Fhotoroom HDR is an easy to use powerful image editor that can take your JPEG's, Digital Camera RAW and High Dynamic Range photos and help you fix exposure, color, and distortion problems or quickly and easily make your photos stunning.
Artweaver is a simple Freeware program for creative painting, i.e. Artweaver offers you all artistic effects which you need for your work.
Photoflare is a cross-platform image editor with an aim to balance between powerful features and a very friendly graphical user interface. It suits a wide variety of different tasks and users who value a more nimble workflow.
WidsMob Retoucher is an excellent photo retouching program, which is able to reduce image noise, portrait retoucher, film pack, adjust effects, fish eye effects, vignetting and more other features to photos with ease.
Pixelot provides an easy method for blurring or censoring regions of a photo with pixels. Use this app to hide faces, license plates, names & addresses, etc. Then share your picture on Google+, Facebook, Twitter, email, and more*.
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PhotoFlare is inspired by PhotoFiltre for Windows, but is not nearly as powerful. Worse than that, it crashes very often under Debian GNU/Linux, depending on which features you use. Nevertheless, I use it for simple tasks.