This wallpaper has been made with custom-created brush. To start, we need a leaf. Here is a leaf I found in Interned. You can also bring a leaf from the street and scan it :-)

Crop leaf image as necessary, desaturate (Ctrl-Shift-U).

Invert (Ctrl-I) and adjust layers (Ctrl-L) to increase contrast.

Use Magic Wand (W) to remove white background.

Go to Edit menu - Define Brush preset.

Now, when brush is ready, create new document (it will be our wallpaper). Fill it with solid color.