Here i'll show you how you can turn a normal picture into a raining picture.
I got the idea to create this tut from daisy's image raining picture(Image manipulation competition

)
See HereStep 1: Open the image you want to use,i used a picture from the parliament in Greece(lol)
Step 2: Create a new layer on top of your image and go to filter>render>clouds
and add a filter cloud.
Step 3: Now we are going to add some noise,go to the filter>noise>add noise,see image below

Step 4: Now we'll add some motion blur,again go to filter>blur and add a motion blur,see image below
Step 5: Then go to image>adjustments>levels and put the properties below,see image

Step 6: Change the blend mode from normal to screen

Your picture now,should look like this:
Step 7: Play with the opacity a little(mine is 29%)
And that's it finished!
Easy huh?