Most of the time, it's best to use HTML text as real text if possible. But occasionally, like with banners you'll need to add text to your image. This text, once it's saved as an image is not read as text by the search engines, but merely as just a graphic image.
For this tutorial you'll need to have some basic gimp skills, like creating new documents, working with gradients, adding new layers, etc. We will not be going over these basic steps.