If I understand Corel Painter, it is more a digital painting application & less of a pixel editor/manipulator like Adobe Photoshop.
The only Linux app that I know of that is approaching painting style of functionality is KDE's Krita http://www.koffice.org/krita/
A example screenshot and a blog of one of the main Krita developers http://www.valdyas.org/fading/index.cgi/hacking/krita