In this tutorial I will be showing you some basic photo manipulation techniques in the GIMP which can be applied to not only this project but any number of other projects as well. The results can look pretty cool. Take a look!
I am creating this tutorial with the assumption that you have some basic knowledge about GIMP and can navigate your way around to some degree. If you need anything clarified, definitely leave a comment and I'll try to help as best as I can.
another thing to note is that I am using a modified GIMP. therefore the window is different from the stock version and things may be in different places than you are used to. Hopefully this won't be a problem.
The two images I used can be found here:
Step 1: Adding images/isolation
Begin by dragging the image of the background into a blank GIMP window. This should automatically create a new layer with the image in it. This will be the base of your image. The next step is to drag the picture of the airship into the GIMP window. This should add the airship on another layer. if for whatever reason you can see the layer in the layer browser, but not in your image, be sure to check that the layer is ABOVE the background layer. Now, select the eraser tool and select a soft brush. Begin to remove the sky in the airship image. If you aren't able to get all of it, it's fine. Since it's a ghost ship a little extra haze is just fine.
Also note that a feather brush is very useful in a lot of cases when you need to isolate something because the soft edge blends better with the rest of the scene.