Paint.NET is a free graphics editing program for Windows. One of the main features, in terms of modding for Fallout 3, is that it has built-in support for .dds files, the format for textures that the game uses. There's no need to install any additional .dds plugins as is needed for programs like Photoshop. It also loads very quickly, which makes it ideal for converting existing images in other formats to .dds files.
You can download it from the official website here