Credit to Insanity Prevails.
Here is another Gimp Tutorial.
OK, we'll go to File > New and set our canvas size and OK it. I'm using 400 x 125 (width x height) as usual. Now we need a stock image to work with. Just copy and paste into the file, then click on New Layer to set it as a new layer. Resize the stock using the scale tool or the scale layer option. Now move it as needed.
At this point I did Edit > Copy Visible and pasted as a new layer. I set the gradient option to Blue to Purple and then went to Colours > Map > Gradient Map (this option is under the Filters menu in 2.2). This layer was set to Overlay.
Copy visible and paste as a new layer. Go to Filters > Map > Make Seamless.
Set the layer mode to Lighten Only. Use a soft eraser to erase anything that doesn't look too good (I erased the bottom-right area a bit).
Now make a new layer and solid fill with black. Now get the gradient tool and apply these settings:
Yeah, gradient spam time. Drag out gradients all over the canvas. We need two gradient layers so do this twice. These are the layers I ended up with:
Hide the second one for now and go to the first one. Go to Colours > Colour to Alpha. Click the coloured bar to open the colour select and set the colour to red (FF0000). OK this. Reopen Colour to Alpha and change the colour to yellow (FFFF00). OK it.
Go to Colours > Colourise. Alter the colours to a darkish setting. I just used the default settings.
Set the layer mode to Dodge and erase whatever doesn't look good with a soft eraser.
Go to the other gradient difference layer we did. Reshow it and drag to the top of the layer stack. Go to Colours > Colour to Alpha. Click the coloured bar to open the colour select and set the colour to red (FF0000). OK this. Now reopen Colour to Alpha and change the colour to blue (0000FF). OK it again. Reopen Colour to Alpha again and pick yellow (FFFF00) and OK it. Now go to Selection > Alpha to Selection. Invert selection (Select > Invert) and delete once (Edit > Clear). You should now have some random coloured mess over your stock that is mostly green.
Set the layer mode to Dodge and use a soft eraser to erase bad parts.
Make a new layer and fill with black. Go to the gradient tool again and change the shape to Bi-Linear. Leave all other settings as they are. Now drag out gradients in more-or-less in the same direction over this canvas. We want lines that flow a specific direction. Like, how Saber is slashing downwards and slightly to the right in my stock.
OK, but this covers the whole canvas. Grab the free select tool and roughly select a "stream" that flows with the action. Feather the selection by a set amount (I used something like 30). Invert and delete selection.
Set the layer mode to Addition. I also used the Colours > Colourise option to change the colour of this layer to something that fit in better (pink, in other words).
Copy visible and paste as new layer. I now set up a blue to pink gradient in my gradient dialogue (I have a custom gradient for this but you can just set the foreground colour to a pink, the background colour to a light blue and set the gradient to FG to BG (RGB)). Now go to Colours > Map > Gradient Map (found under Filters in 2.2). Set the layer mode to Hue at 30 opacity.
Copy visible and paste as new layer. Go to Filters > Enhance > Sharpen. I used a setting of 30, but use whatever looks good. Then free select around the focal, feather the selection by something like 35, invert and delete selection.
Copy visible and paste as new layer. Change the gradient in the gradient dialogue to the one named Default. Run the gradient map filter. Now make a circle selection from the focal outwards to cover a good portion of the canvas. Now feather this selection by a good amount (like 100) and delete selection. Lower opacity as needed (like to 70 in my case).
Duplicate the layer and change the layer mode to Multiply. Lower opacity as needed (I reduced it to 40). Then add a border and you're done.
*Note* - The colours you should apply to your version will depend entirely on the stock you're using. Feel free to deviate away from the colours I used in any step if it isn't looking right for you.