mtPaint is a simple painting program which I developed from scratch so I could easily create pixel art and manipulate digital photos.
It uses the GTK+ toolkit (version 1 or 2) and runs on PC's via the GNU/Linux or Windows operating systems. Because of its simplicity it can also run on older PC hardware (e.g. a 200MHz CPU and 32MB of RAM).
By default it is in the English language, but there are also translations available for other languages such as Spanish, Czech, French, Portuguese and Brazilian Portuguese.
mtPaint is free software, which means you are able to freely use, modify and distribute it according to the GNU General Public License.
Version 3.21 has new Galician and Russian translations, updated Spanish translation, and several small improvements and bugfixes.

mtPaint Features:
Interface

Toggle the toolbars on or off at the touch of a button.
Horizontal or vertical split view mode.
Pixel grid for guidance at high levels of zoom.
Move the mouse pointer with exact precision using the arrow keys.
Extensive shortcut keys used to improve productivity.
12 Image clipboard.
Up to 1000 undo steps.
Zoom levels between 10% and 2000%.
Pan window for quickly navigating a large image, or at a high zoom.
Browse different files passed at the command line.
Translations: Chinese (Simplified), Chinese (Taiwanese), Czech, English (UK), French, Galician, German, Japanese, Polish, Portuguese, Portuguese (Brazilian), Russian, Slovak, Spanish, Turkish.
Use a pressure sensitive graphics tablet.
Limit memory used for undo images.
Grab screenshots.
Pixel Art

Edit indexed palette or RGB images.
Save and load BMP, GIF, JPEG, LSS, PNG, TGA, TIFF, XPM, XBM files.
Extensive palette features - load, save, sort, edit, posterize, quantize, remove duplicate colours, remove unused colours, colour protection, palette shifting.
Image channels: Alpha, Selection, Mask. All saved to a PNG file.
Up to 100 layers.
Create animations by moving layers, or shifting palettes.
Create animated GIF's using Gifsicle.
81 brush presets.
100 brush patterns.
User defined gradients.
Copy and paste using rectangle or polygon shapes.
Tools to shuffle pixels, flood fill, draw straight lines, smudge, clone, paste text, draw/fill rectangle/ellipses.
Image Manipulation

Quickly transform image gamma, brightness, contrast, saturation, hue, posterize from a single dialog, and preview the results in real time.
Apply various effects such as invert, greyscale, isometric transformations, edge detect, sharpen, unsharp mask, soften, Gaussian blur, emboss, bacteria.
Use gamma correction on effects to improve the quality of various effects.
Crop at the touch of a button.
Resize the canvas.
Rescale the canvas, enlarging or reducing, using several filter methods.

The license of this software is Free, you can free download and free use this image editing software.