I know there are many ideas how to create a lightning, and scripts, and new things in G'Mic (which I honestly thought looked mostly like the frizzed-up end of a piece of string on most examples). I haven't seen this idea mentioned anywhere. I could have missed it, easy enough to do.

1. Pick an image with lots of things on it. Ought to work best with a landscape, you know, mountains, forest, clouds and so on, not the format itself, although, a landscape format of a landscape image might work. Or create something using the fog filter or similar (yes, I know about this effect, using several layers).
2. Use the "Scissors Select Tool" (might be called something else in newer GIMPs, mine is, as I have stated before, OLD!)
3. Start at the top somewhere (you can rotate the layer when you're done) and click away diagonally (not Diagon Alley this time!) down the image, about a thumb's width between each point.
4. When you reach the bottom, click outside the actual image somewhere, then again at the top (still outside the image) until you reach your first insert point again.
5. Like lightning, hit Enter once to make a selection.

What you do after this is pretty much up to you. I usually create a new layer and make a path out of the selection. Then I can use the "Stroke Path" thing-a-ma-jig and create the lightning object, to do with as needed in my current image. Cutting away the extra bits which were added outside of the actual image, is a piece of cake.

Truthfully, you do get the best results using the real thing. Grab some photos of lightning and use those. Nothing beats reality! Nothing man made will ever replace that, I'm sure.

Like to see your pictures of when and or how you do it. I think david has a good filter for lightning in GMIC and there was one in the gimp Fractals.
I have got some good photos using them somewhere on here.

Everybody ought to be able to point and click and select the tools so I won't go through all that. Do what the text says.
I will provide two images, however. One that shows the step after the scissors tool has been used and made into a selection, the path's been made from that selection, and stroke path has been used on a new layer.

The next image is the finished result. Lightning was rotated and some color added. I painted an origin "cloud" of sorts for the lightning with a brush I made for painting fluffy things.

N.b. this is only a demo of ONE lightning rod. It's just as easy to make several more when one is already at it. Then change size, rotate, whatever, and you can quickly build a very complex thunderstorm.

Time spent: Roughly 15-20 minutes, including finding the image and creating the lightning, writing this, adding images to the upload, removing one of them because they came in the wrong order, and re-upload it.

Image source:
https://www.publicdomainpictures.net/en ... ttlesnakes
License: CC0 Public Domain