When you are replacing a background it is important to shoot with that in mind. Taking an existing photo and just replacing it will not cut it, sorry
Try this again, but with a couple of things in mind:
- The subject should have some interaction with the background to make it more believable. Something as simple as seeing their feet touch the ground can do the trick
-Pick a background that would be suitable, and would not give away the fact that it was not there originally. We all know what wood chips look like, and what size they should be. Placing a wood background like that behind such a tiny girl is telling me either she is smaller than I previously thought, or that would is much larger than normal. Logic instantly dictates that the picture is photoshopped.
- The original photo must have as neutral a color tone as possible, this is extremely important. This will allow you to replace the background, and match the subject better to the colors going on in the background.
- Always plan to take a specific picture with the intent to replace the background later. My other points have somewhat stated that indirectly, though I would like to make this clear.
- The background shouldn't be sharper than any part of the subject
- When cutting your subject out, make sure that you go back and select it, than put an extremely small feather on it to smooth out any harsh lines created during the process.
Hope this helps you out a bit!