Pixel art graphics aren’t that widely used in computers these days. But in the past where graphics hardware weren’t that powerful and advanced, computer games and other graphics were created using pixel art and it used to rock :). It certainly ain’t dead (still being used in lower end cellphones, game consoles etc) and if you’re still quite fond them and looking for a pixel art oriented painting app then “mtPaint” is worth trying.
It comes with a reasonable amount of features and runs in GNU/Linux and Windows. Also supports layers and you can even load images into it for editing which is not always supported in “simple” painting apps :).
Some of its main features …
*. A drawing tool-bar with standard tools such as: A pencil, Fill tool, Eraser, Cloner, selection tools, Gradient tool, insert Text, Flip etc.

*. Supports saving images in PNG, GIF, BMP, TGA and TIFF Formats.
*. Advanced Clipboard support (storing up to 12 items).
*. Zoom in/out (up to 2000%).
*. Undo/Redo support (you can manually change the reserved memory for undo operations).
*. Built in different brush types.
*. Free rotate and flip images.
*. Add, Save, Delete layers.
*. Few built in image effects (Greyscale, Bacteria, Edge detect, Soften, Sharpen etc).

*. A color palette editor.
*. Change image overlay settings… are some of its features to mention.
You can install mtPaint in Ubuntu 12.04 Precise Pangolin, 11.10 Oneiric Ocelot, 11.04 Natty Narwhal, 10.10 and 10.04 by typing the below command in your Terminal window.
sudo apt-get install mtpaint
That should do it.