HOW TO SET A SKY BACKGROUND IN BLENDER (memo)

MEMO: set a background image in Blender 2.5

The problem is that many tutorials say how to set an image on the blender background's, but like a model, not a scene sky.

So:
Go to:
[textures]
(category World)
add a texture, type "Image or Movie", open your image
Go to ->Influence (the fuckin step that wasn't specified) and set "Horiz" horizon and Zenit, the default value is 1, do your own tests.
This is also "Blend:" which is set as "Mix" as default, you can change it, for exemple by Multiply, or add.

If you go again in [World] you can change the colours "paper", "blend" and "real" and see wtf.


Observations:
[Texture->World->Mapping->Coordinates] : or a classic image (plane) or a AngMap, or I dunno. AngMap: like a camera view, spheric.