Firstly, I also feel the same, some of them actually have good pictures and also a scheme for sure. However, not all of them have high resolution.
Some ideas above can be really applied to your wallpaper. They are nice enough.
However, I agree with some suggestion, if you can edit some pictures, the results may be really better for you, including suitable enough to what in your mind.
You can make it using Photoshop or correl.
Well, reading this thread makes me also want to make some wallpapers.