How can I change my object material to an image texture? I want to be able to export a blender material applied to an object as (for example) a .png file. Is this possible ?
One way to do this is if you are using the default internal render engine is to generate a UV Map from an object hosting the material and then bake the texture to an image.
Open the UV/Image Editor and create a new image (you can set the output resolution from here), next select the object, Tab into Edit mode and press U to generate a uv map, for very simple models you can use the default Unwrap or Project from View (for more complicated objects it's also best if you use a simple object with the material on it as opposed to unwrapping a detailed mesh) go to the Render tab and under Bake, you can set the Bake Mode to Textures (if you are using a custom texture) or just use Full Render.
In the image below, I created a simple green material and added a wood texture to it and the did a Full Render bake, the result is shown in the Image Editor on the left. After that, use the Image menu to save it.