mtPaint is a GTK+ painting program which Mark Tyler developed from
scratch. Currently mtPaint is maintained by Dmitry Groshev. It is
aimed to be simple and easy to use. 

mtPaint can edit indexed palette or 24 bit RGB images and offers basic
painting and palette manipulation tools. It also can handle JPEG, GIF,
TIFF, BMP, XPM, and XBM files.

NOTES:
If you want this to build as a GTK3 app, pass GTK3=yes. mtPaint comes
with its own file and color selector widgets. To use the GTK2 default
ones, pass TKWIDGETS=yes to the script. This will have no effect with
GTK3=yes.