Create a fake sun

1. First open the image you want to add your fake sun to and convert it to a layer by holding down Alt and double clicking on it's name in the layers palette. (arrowed)

2. Create another layer above your image layer (Shift+Ctrl+N) and change it's blend mode to screen (arrowed), fill this layer with black.

3. Make sure the top (black) layer is selected and go to Filter>Render>Lens Flare, use the settings shown.

Try and get the flare as close to the center as possible, this is so all the rings in the flare line up.

4. Next we need to apply Radial Blur, so, Filter>Blur>Radial Blur. Apply the setting's shown making sure the Zoom radio button is selected.

Once the filter has been applied press Ctrl+F to re-apply it, do this a further two times.

5. We now need to feather the Lens Flare so that the edge's are not visible when we move it to the correct position in the image. To do this make a large selection (using the Elliptical Marquee Tool) around the flare, holding down Shift while doing this will make a perfect circle.

6. Go to Select>Feather (or Ctrl+Alt+D) and enter 100 pixels. The amount of feather depends on the size of the image you are working on - a larger image will need more feathering and vice versa. Once you have set the feathering amount press Ctrl+Shift+I to invert the selection and press Backspace 2 or 3 times.

7. Once you have de-selected (Ctrl+D) you can move your fake sun to what ever part of the image you desire. If you want a brighter sun just duplicate the Lens Flare layer. If the edges are visible use a soft edged eraser to clean up any visible lines. Finally flatten your image and save.

That's it, simple but effective.

Before

After

This also works great on space scenes and you can change the colour of the flares using Hue and Saturation (colourize box checked).