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
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.
Step 2: Scale and Move
Step 3: Making It Ethereal
Next, in the top bar third from the right is a button called filters. Click on this. In this drop down menu is a button that says blur. hover your mouse over this and then when the next menu pops up click on Gaussian blur. In the window that appears make sure the link of chain next to the two input areas is connected rather than broken and that the units are in pixels. Now in one of the input fields type in something close to 20 pixels depending on how much of an outline you want. Click OK and let it process this.
Now make sure your new airship layer is selected and at the top of the layer browser look for the box called mode. (next to it it will say normal.) Click on this and in the drop down menu select dodge.
Now, with the blurred airship layer still selected, go to the top bar again and click on the button called colors. It should be five places from the right. Now click on Color Balance. In the window move the bottom slider towards the yellow, and the middle slider towards the green until you like the hue. Now go back to where you selected the layer mode and slide the layer opacity slider down until you like the effect.
Step 4: Two New Layers
Now duplicate this new layer and repeat the process, moving the layer down and to the right. Then adjust the layer opacity again as necessary.
Step 5: Final Touches
Now for the background. This is purely a personal choice but I didn't like how dark the background was. So I selected the background layer and clicked on the colors tab again at the top of the page. Then I clicked on brightness and contrast and lowered the contrast a bit to take the edge off the clouds. And that's it! you're done! Now just click on File>save as and then name your file whatever you want. Make sure that after the name you put a file extension. (for instance, .jpg or .png) now click save and okay to any messages that pop up.