Use a drop shadow layer to darken an image.
After I put a huge drop shadow on the wall, I deformed it, making it
longer and larger. You can crank the size up.
It was easy to crop the bits I didn't want.
PSP version 8 has a check box that lets you put the shadow on a
separate layer. You can even apply more than one shadow.
Make your own experiments. You can apply more of a Gaussian blur
to thin it out or make the visibility of a layer less..