If you’re an artist, you’ll probably find many image editing applications are tool pixel-orientated for natural painting. There are a few natural painting programs available, but they are rather limited and most image editing application are geared towards post-image manipulation, rather than creating your image from scratch.
Artweaver is an application geared towards artists capable of painting through a pen tablet or directly on to their screen, via their mouse. It contains a range of brushes and the brush simulation is as realistic as possible, with chalk, charcoal, pencils and other brushes included.
In addition to its natural painting, it will enable you to make sketches from existing photos and also add effects to images. It will enable you to import from a range of image formats.