You can view original wallpaper here:  Diving Wallpaper (1024x768px)

First step is to create blurred grass in the background. Set foreground color to dark green, background color to black. Then switch to gradient tool (G), make sure that current gradient is green-to-black, select Radial gradient (on top bar), then draw a big gradient from center to the edge of image.

I found a stock photo of grass and decided it will be good for background. Paste the image into your file...

... then resize it to fill entire image, desaturate (Ctrl-Shit-U) and apply Gaussian Blur (found in Filters menu - Blur) with large setting (50-100 pixels).

Set this layer opacity mode to Overlay.

Now let's create some black leaves in the background. It is very easy - set foreground color to black, then use Pen tool (P) to create some simple shape of a leaf.

Create some more leaves, then set their oapcity mode to soft Light and opacity to 30-40%. They are only for background and should be barely visible.

Now we need the main leaf, where the story will happen. Set foreground color to vivid green, then in the new layer, use Pen tool to make the shape of the leaf (I wanted it to twirl a bit).

Here is how the completed shape layer looks.