Working with Field Images

I have created a house in Blender. Now I want to apply a texture. I have all the textures as a PNG file, but all the textures are in the same file, hence an field image.

I understand that Blender supports Fields. I am able to load the image into Blender and to assign it as Fields. But where and how do I pick the coordinates of the field inside the image? I do not understand this and did not find anything on the net. Maybe I am just looking for the wrong term.

Probably a straight forward answer, but help is really appreciated.

If I understood this right you are looking for UV mapping? Try loading your .png file in uv/image editor and work your way from there.