Open the skyline texture in GIMP. Use the Fuzzy Select tool to select the sky.
Click several times if you need to.
Go to Layer ¦ Transparency ¦ Add Alpha channel.
Now click on Delete or go to Edit ¦ Clear.
Go to Layer ¦ Transform ¦ Offset and offset the image by 1500 in X as shown next. This will wrap the edges round to the center.
Now, use the Clone tool or Smudge tool to make the center seem seamless.
You should have something like the following. Save this as a PNG file named Backdrop_Skyline.
What just happened?
You created an Alpha channel which defines which pixels are see-through, and which are visible. You then made the sky area see-through so that you will be able to see the sky through it in Unity. You made the image seamless so that it can wrap around a cylinder or square.