If you want Free, GIMP is a good go to for now, but it you want value, I vote for Pixelmator – stuffe Mar 20 '12 at 15:11.
Apple encourages customers who need X11 to visit the site and download XQuartz version 2.7.2 or later. However, X11 server and client libraries for OS X Mountain Lion are available from the XQuartz project.
As some of you may have already heard, X11 is not included with OS X version 10.8 aka Mountain Lion.