Here is a quick and easy way to create realistic lightning in Adobe Photoshop.
Step 1. Create Your Brush
I'm going to assume you have already created a new image with a dark background. So, lets get to making the brush. Select the Pencil tool and make a small, sharp brush using a size of 1px and hardness of 100%.
Step 2. Draw Your Lightning
Drawing lightning isn't an art. Keep it jagged. Make sure you have created a new layer and selected white (#FFFFFF) to draw with. When you are done, you should have something like this.
Step 3: Apply a Glow
Open up the Layer Styles window by double-clicking on the layer with the lightning on it. Go to 'Outer Glow' and create a glow with the following settings:
Blend Mode: Screen
Color: #7e90ff (you can try other colors if you wish)
Now you should have something like this:
That's really how simple it is. With a bit of practice you will be able to add this lightning to any photo.