I have been asked to make a tutorial outlining how to make the backgrounds images like this:

I call these ‘Mirror Backgrounds’, as they resemble a faint image of the character itself, as though looking at them in a mirror. They were inspired by, but not copied from, the character screens on the computer game Sonic Adventure 2: Battle.
As always, I’ll be using Gimp 2.8 in this tutorial, but these steps are pretty much universal to all versions of Gimp.
Step 1:
Prepare the image you want to use as the foreground picture. If you don’t already know how, follow the steps in
this guide to learn how to colour your character in using Gimp.

Step 2:
Make sure your background is plain, and go to ‘Layers’, ‘Transparency’ and click on ‘Add Alpha Layer’, if available. Then, using the ‘Fuzzy Select Tool’ in the Tool Box, click on the background. Once it’s activated, erase the background with the ‘Eraser’ tool, so it’s transparent. For a detail guide to this step,
go here
Step 3:
Next, go to ‘Layers’ and select ‘New Layer’. Then, in the ‘Layers – Brushes’ box on the right hand side of the screen, left click and drag the new layer underneath the layer with your character on it.

Step 4:
Next, go to the ‘Tool Box’ and select the ‘Blend Tool’. This looks like a little square that is gradually fading from grey to white. Set your primary and secondary colours to whatever you want, although I find a colour and white work very nicely for this.

Step 5:
Then, click and drag the curser from one corner of the image to the opposite corner, going diagonally. Once you release the curser, a graduated background will fill your picture, fading from your primary colour to your secondary colour. Experiment until you find something you like.

Step 6:
Click on the layer with your character, right click on your character and press ‘Copy’. Now, repeat Step 3, dragging the new layer between your character and the background. ‘Paste’ your copied image onto this. I should paste directly behind your character, so you won’t see it except for the blinking dotted line around it. Next, go to the ‘Toolbox’ and click on the ‘Scale Tool’.

Step 7:
With the ‘Scale Tool’, click and drag the middle layer character. You will notice a grid appear onscreen, and you can make the new layer as big or small as you like.

Step 8:
Release the ‘Scale Tool’ and use the ‘Move Tool’ to move the middle layer around until you’re happy with it. Next, go back to the ‘Layers-Brushes’ box and reduce the ‘Opacity’ of the middle layer, making it fade out. At least 50% Opacity is a good place to start, but play around until you’re happy with it.

Step 9:
If you want, you can also go to ‘Colours’, ‘Brightness – Contrast’ and increase the brightness of the middle layer, further paling/fading it.

Step 10:
Once you’re happy with the image, right click on the layers in the ‘Layers – Brushes’ box and click ‘Merge Down’, starting at the bottom layer and working your way up. Or you could simply go to ‘Image’ and click on ‘Flatten Image’.

And that is how to make my Mirror Backgrounds ^^
Render (c) me
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Huge thank you to Roiffalo for help with sorting out my signature ^^