The C++ class library: http://www.paintlib.de/paintlib/paintlib-2.6.2.zip (about 1,8 mb) or http://www.paintlib.de/paintlib/paintlib-2.6.2.tar.gz (about 1200 kb).
Download this file. In addition to it, windows users need to download libcurl, libpng, zlib, libtiff, libungif and libjpeg for png, tiff, gif and jpeg support. See the links page for locations of these libraries.
paintX: http://www.paintlib.de/paintlib/paintx.zip (about 200 kb)
Older versions of paintlib are also still available, starting with version 2.0:
The latest development snapshot is available via anon cvs at http://sourceforge.net/projects/paintlib/