I'm going to attempt to tell you in simple terms how this is done...hopefully it will be easy to understand..:)
1. Open up a an image of some scenery.
2. Open up a tube you want to place "into" the scenery".
3. Open up a blank image of about 500x500 (I like to have lots of room to work)
4. You can minimize the tube that you will be adding down to the bottom of your desktop for now to get it out of the road.
5. Now, flood fill your blank image with a coordinating color from your scenery image....to do this...click over on your paint palette and you should see a dropper at the end of your cursor. Then, move the dropper over to your scenery image and touch it down to a color in the image. This will put that chosen color into your paint palette. Then, just click on your paint fill tool and flood the image.
6. Next, go up to layers and add a new raster layer. (the reasoning for this is so if and when you want to lower the opacity on your scenic picture to make it dim, you wont just see the little black and white squares behind it.)
7. click the top of your scenic image and go up to edit and down to copy.
8. Click the top of your flood filled image and then go up to selections and down to select all. (this will give you marching ants around your image)
9. Go up to Edit once more and down to "paste into selection". this should have placed your scenery into the new flood filled image.
10. Go to selections, select none to remove the marching ants.
I'll continue in the next box...:)