If your into making professional looking posters then try
http://www.osalt.com/photoshop
"Open source Photoshop alternatives
Paint.NET 3.05
Available for: windows mac linux unix java
Paint.NET is a .NET open source bitmap drawing and editing application. One of the main goals of the project is to provide a simple and easy to use application with an intuitive user interface.... Read more
CinePaint 0.21
Available for: windows mac linux unix java
Started as a fork of the Gimp project, the goal is to create a more professionel graphical editing application aimed at the movie industry. One of the major differences from Gimp is that CinePaint... Read more
Gimp 2.2.13
Available for: windows mac linux unix java
Probably the oldest and most well-known open source graphic application - GNU Image Manipulation Program or Gimp was started in 1995 and has since then grown to the status it has today. Gimp is a... Read more
Gimpshop 2.2.11
Available for: windows mac linux unix java
If you feel at home in Photoshop, but need an open source alternative - Gimpshop is the way to go. Gimpshop is a hack made on top of the original Gimp project which changes all menues, dialogs, etc.,... Read more
Krita 1.6.1
Available for: windows mac linux unix java
Krita is open source bitmap editing and drawing with the following advanced features: 16-bit image support, High dynamic resolution images, CMYK and even color management. As part of the KOffice... Read more"
Hope this helps