You did a decent job. I wasn't trying to put you down. Most people probably wouldn't notice. I work with photoshop all day, so I catch things like that. lol.
Here's a few things that would have made it more believable:
-The sky showing in the fence is relatively dark, so I would darken the clouds some. Also, I would remove some of the contrast for the same reason.
-On the left side, there's an obvious split in the clouds that should result in some of the blue sky being visible through the fence, so I would add some blue sky there. If you had done this, I probably would have had trouble determining whether the sky was real or fake.
Here's how it looks with those two things done:
And I still might have brought the contrast down some, or up the contrast on the fence.