Based on your sample image, I would say that you have to imagine the wall stopping the light. So on the tile for the wall, it would be lit, but past it (moving away from the light source) the light would not continue. In your example, the top tile of the emitted light (2 squares up from the source) would be dark but the wall tiles would stay the same.
Now, that said, if you have different layers, you may want light from a higher layer to transfer down a wall to a lower layer. But that depends on what you want and what looks good.
I guess the idea is, imagine yourself in that room, with that light source. What would happen when the light hit the wall? It would stop at the wall, right? Also, if you want to go wider with the emitted light, try and imagine the shadow it would cast as it passed the right and left edges of the wall. It would go off on an angle from there so maybe on the tiles that it crosses, you'd have partially lit tiles there.
Something you may also want to consider, if you haven't already, is that if emitted light overlaps, you may want it to get brighter. In that case, you'd want the light effect to be additive (i.e. the "light value" for a tile from one light source is ADDed to the "light value" on the same tile from the other light sources).