Heres a nice and easy way to create rain to ur sigs or walls!
Step 1..
Add a layer to the sig u are making rain to..make this layer all Black.

Step2..
You now go to Filter>Noise>Add Noise..and apply these settings as shown..

Step 3..
You now go to Filter>Blur>MotionBlur..and apply these settings a shown...

Step4..
You now change the "rain" layers settings to "overlay" and change the opacity of the layer to 30 % as shown below..

Step5...
Now repeat steps 1-4 another 4 times (you can make more layers if u want) but each time you make a new rain layer make the motions blurs angle percentage 1 to 5 % more or less... (Ex. Layer1=60%, Layer2=65%, layer3= 63% and so on) (you can make as many rain layers as u want)
Step6...
Now go to imageready...
Step7...
Once in image ready if the animation window isnt already up go to window>animation.
Step8...
now in the layers pallete uncheck all the eyes(layers) except one and the background layer.

Step9...
now add a layer to the animation window..and uncheck the first rain layer and replace it with another one of ur rain layers u made
Step10...
Repeat Steps 8-9 untill your satisfied with the way it looks..! (mess around untill u got the way u want it..includes deleting layers and adding layers)
Step11...
now press play and see what it looks like, if done properly it will look like its raining in your sig.
YOUR FINISHED!! Hope you enjoyed the tut..

Here mine

Heres also another one I made using the "rain" effect