Is there a "light" version of GIMP?

If you're on Windows, try Paint.NET

If you're on OSX, have a look at Seashore. From the website:

Seashore is an open source image editor for Mac OS X's Cocoa framework. It features gradients, textures and anti-aliasing for both text and brush strokes. It supports multiple layers and alpha channel editing. It is based around the GIMP's technology and uses the same native file format.

However, unlike the GIMP, Seashore only aims to serve the basic image editing needs of most computer users, not to provide a replacement for professional image editing products.

I would call that a 'light' version ;-)