The best open source alternative to GIMP is Krita. If that doesn't suit you, our users have ranked more than 100 alternatives to GIMP and many of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to GIMP are MyPaint, PixiEditor, PhotoGIMP and Fossify Paint.
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.
MyPaint is a fast, distraction-free, and easy painting tool for digital artists. It supports graphics tablets made by Wacom, and many similar devices.
Create pixel-perfect sprites, raster paintings, and scalable vector graphics with a unified open-source editor featuring frame-by-frame animation, a node-based workflow, custom shaders, procedural art tools, anti-aliased brushes, and versatile export options.
Privacy-centric digital drawing tool perfect for all skill levels. Create, customize, and share art with adjustable brushes, canvas colors, and versatile format support via PNG, JPG, and SVG without compromising your privacy—fully open-source.
Raster & vector graphic editor with many tools and filters, written in Lazarus. Portable, 32bit & 64bit versions, ORA format & 16384x16384 image support. In app 3D import & rendering.
miniPaint is free online image editor using HTML5. Edit, adjust your images, add effects online in your browser without installing anything. You can use the Electron based version of the editor if you like.
BSD-licensed portable photo editor for Windows featuring multi-layer editing, batch processing, macro support, real-time effect previews, advanced filters, customizable hotkeys, non-destructive tools, color-managed workflow, efficient performance, and broad file compatibility.
AzPainter is a full color painting program for Linux. It's a lightweight tool with tablet pressure support, and provides basic functions such as selection, layers, filters. you can use it for painting, image editing, or pixel art making.
Open source image editor with non-destructive editing, 110+ filters, adjustment and text layers, support for smart objects and adjustment layers, cross-platform compatibility, and layer-based editing for flexible image composition.
Pocket Paint is a paint editor that, among others, allows setting parts of pictures to transparent and zooming up to pixel level.
GPS (aka Gimp Paint Studio) is a collection of brushes, patterns, color gradients, accompanying tool presets and dynamics. Tool presets are a simply saved tool options, highly useful feature of the GIMP.
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I disagree with most of the negative comments because you can actually do image manipulation/photo editing as well, at least the basic stuff like auto contrast, color balance, saturation, blur, sharpness, image size, canvas size and so on.