08-13-2010, 03:07 PM
Both of these images have a lot of issues. Rather than point each one out individually, which doesn't necessarily help in a broad way, here are some things you need to think about:
1. Duplicated elements (such as those created by a clone brush) are always obvious, and need to be completely avoided.
2. Cloned areas with blurry edges are even more obvious.
3. All objects should share the same common horizon. I know there are sometimes exceptions to this, but you can't properly break a rule unless you know how to follow it first.
4. Make sure your light angles and shadows match. You are allowed to have a scene with flat lighting and diffuse shadows, but it will always look boring.
5. Make sure objects that should bed into your scene don't stand out because you haven't matched colour grade, exposure, level of haze, grain, blur, or black levels.
6. Pay particular attention to the edges of things - it's one of the things we notice most. If an edge is too straight, too sharp, or cuts off an element it shouldn't, it will stand out.
Hope that helps