Ever wondered how they get those drops of water to look so realistic? Its time to give your images a baptism!
Open background image. Here we used 400 X 250 pixels at 72dpi.
Copy this image to your computer or use your own.
Create a new layer. Press "D" to set default foreground
and background colors. Filter > Render > Clouds.
Filter > Stylize > Find edges.
Now adjust the levels for more definition in the image:
Image > Adjust > Levels or (Mac:Command+L / Windows:Ctrl+L)
To create the drops: Filter > Sketch > Plaster. You will
have to experiment to find the best settings for your image size and intensity
of the effect, here we used: Balance 40, Smooth 14, Light - Top Left.
The result should resemble this
To define the edges a bit more: Filter > Sharpen > Unsharp
Mask: Amount 500, Radius 1.0, Threshold 0.
Select the magic-wand tool, Check the contiguous box
on the options bar.
Set the appropriate tolerance setting, we used 10. Now select the
background around the drops.

Hit the del key to erase the unwanted background. Invert the selection to the drops. Select > Inverse or Mac:Cmd+Shift+I / PC:Ctrl+Shift+I. You should now see the original image and the drops.
If you wanted the image to appear as if mercury had been spilled on it you can stop right here. For the water - proceed...

To make the drops look more like water change the layer blending mode to soft light.
We now have an image that looks like water drops on it. But lets have some fun and give it that lens-type distortion typical to real water drops
With the drops
still selected, click the background layer on the layers palette to activate
the background layer.
Make the distortion: Filter > Distort > Spherize. 35%
(experiment to find what best works for you.)
You
now have an image that looks like it was left out in the rain with realistic
water drops. Play around with it and see what else you can come up with
here is a graphic example of blood