Cannot light up a basic 2D image background sprite.

Hello,
It's the whole night that i'm trying to do a suuuuper basic step in lighting a scenario.
I have a background image and two sprites in the layer upon.

I changed the material of the background and the sprites to Sprites/Diffuse.
I changed the z-axis of the light, to have it between camera and the background.

BUT STILL i see the whole background image not lit.
The strange stuff is that the two sprites upon are correctly lit.
So i can think of a problem about the background image, but it's a normal image. really basic stuff...

I really don't know what to think. Please help!
Ask me whatever you want about the project, i will answer as much as I can

 

Other strange stuff happens..
The first image is when the light has a normal intensity (1). The background is not lit, but the sprites are (you cannot see that from the image, but they are)
The second image is when the light has an extreme intensity. What happens? what is that holy light beam?

 

And last, this is the background image. It has the same material of the other sprites