Added: This tutorial was inspired by theharvester's recent post at the Monthly Screenshot (May).
First, look for a nice wallpaper, preferably a landscape or cityscape. It might take a bit of looking at the result to decide whether the effort was well worth the time. Resize the wallpaper to the desired resolution.
Next, download the following two filesGryphen's Dark 2 without the original logo
and Geminiguy's PCLOS logo
Originally posted as Ruel24/Sproggy's. Apologies for the mistake, geminiguy.
Open the wallpaper you want to work on. Also open Gryphen's Dark 2 and resize it to match the resolution of your wallpaper, and locate the layer and drag it to your wallpaper window. Close the original Dark 2 window. On the working wallpaper window, Gryphen's Dark 2 should cover much of the area and should be perfectly centered. On the top menu, click Layer > Transparency > Color to Alpha. On the dialog box, click the color after From:/before to alpha and and the color palette should popup. Select the darkest color. Click OK and the layer behind should become visible with the industrial grate.
The PCLOS logo is optional. Use it and be proud. All you need to do is open the file in GIMP and drag the logo layer to your wallpaper. Again, it should be perfectly centered.
Remember to use Save As when saving your work. At least this way, you'd get to keep your original wallpaper.