[RM2K3] QUESTIONS ABOUT USING PICTURES AS LIGHTING

Hi all , I want to be able to use a picture to overlay over the map to add extra lighting like sun beams coming from windows and street lamps shining bright , stuff like that ,But I'm finding it hard/confusing to find the best way to create a picture of right size for each of the different maps for some reason It's just confusing me how to get each maps size correct so they fit on the screen properly.
There is a guide here by Lotus but it doesn't show you how to step by step in pics how to make a map look like this image I uploaded as an example.
The map I am trying to do first is 30 by 40 in the map screen so should I make the picture that size ?