Pinta is a free and open source drawing and image editing program for Linux, Mac, and Windows. It is a light weight Paint.NET image editing application. It comes with freehand drawing, layers support, color picker, lines, rectangles, ellipses, lasso tool, magic wand tool, clone stamp and recolor. It comes more 35 effects and adjustments settings like black and white, invert color, sepia, curves, pencil sketch, oil painting and ink sketch.
It is translated into over 55 languages and it supports full unlimited undo. It is a simple and easy to use application. You can download and install it via below download link. It is released under MIT License.
For the Ubuntu Linux installation instructions follow below link