Photoshop - How to Add Light to an Area Using Overlay Blend Mode
Finished Image - This image was created in E-on Software's Vue 6 Infinite. I added the lightning in post work in Photoshop CS2 using Alien Skin's Xenofex 1.0. After I did, I noticed that some areas around the lightning should be lighter. I lightened the sky around the image and, then, decided to lighten other areas such as the rock face that is nearest to the lightning. Here is the finished image.
This is what the original rock face looked like. It was very dark. The Blending Mode for this layer was Normal.
Step 1 - First, I decided the area I wanted to lighten. It is the area surrounded by the white line. The white line is only in the tutorial to show the area.
Step 2 - On a new layer, I chose an off white and painted the area to be lightened. The Blend Mode is still Normal.

Step 3 - I changed the Blend Mode for that layer, Layer 6 copy, to Overlay. Ignore the opacity percentage for this step.

Note: Layer 6 copy did not have to be a copy. I could have "painted the light" on a new layer.

Also, this technique will not work if the layer beneath the "painted light"( in this example it would have been layer 6) was black.

Step 4 - In Overlay Blend Mode, it looked like the image on the right. The layer opacity is 100%.

Step 5 - I changed the layer opacity to 25% (as seen in the screen capture, step3).

I, also, did the same procedure for other parts of the image that I wanted to appear lightened by the lightning.