Now merge all layers (Ctrl-Shift-E), it is a good idea to save a copy of PSD file before that. For resulting single layer, open layer effects, and add Gradient overlay:
- blend mode = linear burn;
- opacity = about 20%;
- gradient = white to black (reversed);
- style = radial;
- scale = maximum.

This will give us nice fading effect, where image is darkened at the edges.

I thought it is nice touch to add subtle water drops to the image. Here is how to create them: make new document, pretty small (100x60 pixels as example). fill with grey.

Go to Filters - Noise - Add noise, apply very big noise (200%).

Here goes tricky part:
- switch to quick mask (Q)
- repeat noise effect (Ctrl-F)
- turn off quick mask (Q)
- delete selection (Del).

Repeat previous step again...

..and again, until black dots are not too close to each other.

Resize image to match the size of our wallpaper (for me it was about 1000%).