mtPaint - Graphical Editor For Linux.
Mtpaint is a free graphical editor based on open source (GNU) that uses the gtk + toolbars. It is aimed at creating indexed palette images and pixel art. The program was originally developed by Mark Tyler.
Due to its efficient design, it can run on older PC hardware (such as a 200mhz cpu and 32mb frame). It can edit indexed palette or 24 bit RGB images and offer basic painting and palette manipulation tools. It aims to be simple and easy to use. It can also handle jpeg, gif, tiff, bmp, xpm and xbm files.
Latest version: 3.50 2021-01-01 download here.
How to install via a terminal window:
This will install an older version: 3.40
sudo apt-get install mtpaint