Paint Shop Pro v7 » Rain
This was an effect i learned from a girl on hl actually. It doesn’t fit all pictures – but you can try on a background pic you want to use, and look critical on it after you added the effect to decide if you want it on or not.
I’ll use this background picture:

Duplicate the background picture. Open the layer palette by clicking L on your key board. Then right click on the layer name in the palette and choose duplicate. You now have a layer called copy of layer 2.

Go to “effects” in the top tool bar, and choose “noise” then “add”.

I chose noise 50

Now your pic looks like this

Go to “effects” in the top tool bar, and choose “blur” then “motion blur”

I chose angle 167 and pixels 30.

Now my pic looks like this

Go to “colors” then “colorize”

I chose the grey color by setting the hue to 0 and saturation to 0

My pic now looks like this

Then go to the layer palette or go to layers in the top tool bar and reduce the opacity on the layer that has the rain on

And the result is:

If you want a horse on, you can do the same effect on it :)