If you've ever had a picture of a musician that could benefit from looking like a concert image then you can recreate a concert's lighting quite easily using Gimp. Even if you have taken a picture of someone playing a gig, sometimes it looks more like they're rehearsing in their bedrooms rather than performing to a crowd. This method can therefore be used both to remedy photographs that didn't turn out how you hoped, or to try and convince people that you really did play that gig in front of a screaming throng of paying customers.
Step 1. Open your image and duplicate the Background layer before renaming the new layer so it's easier to differentiate between the original image and the edited version in the Layers Pane. This is done by right-clicking the layer in the Layers Pane and selecting Duplicate Layer then right-click the new layer and select Rename.
Step 2. Now select Filters, Light and Shadow and Lighting Effects to open the Lighting Effects window. This allows you to create a custom lighting effect or load a pre-configured one.
Step 3. Underneath the preview to the left of the Lighting Effects window there should be a tick-box; make sure this is active to allow you to interact with the image. Grab the blue dot and drag it to the area you want the light to point at; note that this works almost as if it is in 3D space, so if you move it towards the edge your light will become elongated as if it is coming from outside of the image, and if you place it in the middle you will get a smaller and brighter area affected almost in a perfect circle. You can reduce the Distance from the centre of the image that the light is by manipulating the slider in the right of the panel; otherwise most of these settings can be ignored in this task.
Step 4. We can now customise the light further by selecting the Light tab. Select the block of white next to where it says Colour, this will allow you to set a custom colour for the light. My local venue seems to use mainly a set of magenta, cyan and yellow lights, so I'm going to recreate that here, firstly we'll set a magenta one. Play with the intensity setting; the higher the value the more vivid the light will appear and the brighter it will appear as a result. You don't want this setting too high if you want your image to appear at all realistic in the end, so be careful with this setting. Feel free to explore the rest of the settings, but here we only need these first two tabs of the Lighting Effects window, so you can press OK to apply the lighting and you will be presented with the result.
Step 5. Now, if you want to have several lights effecting you object, you'll need to duplicate the background image once again to create a layer for each light source you want to create. So do this and rename the layers so that you can remember which light is going to be in which layer. I'm having one for a yellow light and another for a cyan effect, so create these lights as you did before in their respective layers.
Step 6. Now you will only be able to see the top-most layer, so we need to alter the layers to create the right balance of lighting. Firstly, we have to diffuse the lights, so change the layer mode of each light to Addition and reduce the opacity to around 60% (or whatever makes them suitably translucent for your image).
Step 7. Now it looks more like our image was of a gig performed in the middle of a burning building rather than a concert venue; everything is far too bright. So duplicate the Background layer one more time so we can begin to correct this. Rename this layer and open Colours, Brightness/Contrast. What we have to do here is remove almost all of the brightness in the image and then adjust the contrast to help maintain the detail of the original image.
Step 8. Finally, tweak the opacity of each of the Lighting layers until you get the balance of lighting that you want. Then duplicate the Background layer one more time if the image is still too bright and place the new layer at the top of the Layers Pane. Change the layer mode to Overlay or Multiply depending on what works best and then reduce the opacity until the lighting shows through and the image is still darkened enough to look correct.
There you have it, that's how you create stage lighting effects in Gimp.