There's another way you can achieve lighting.. this way doesn't require huge sprites and you can have multiple lights too:
If you create a layer above your current and name it "lighting"
Have these settings:
background colour > black
Transparent > no
Opacity > 100
Force own texture > Yes (not entirely sure what this does, but it doesn't work without it)
Visible in editor > no (otherwise you can't see sh*t :) )
And then the actual 'light' object needs to have the effect called "Destination out" (in the object properties) and also needs to be on the "lighting" layer.
The 'light' objects image itself needs to be just be white with transparency (can be a bit awkward in some image editors)
The more transparent it is, the less it 'erases' the darkness if that makes any sense.
Here's an example of a light i'm using:
Try highlighting it or copy it into an image editor if you can't see it.