Okay, this one is directed towards the GIMP-challenged, so forgive me when I repeatedly give too much direction:
To put the rain effect in a GIMP sig:
*Step 1: Take the sig you want to add the effect to, and make it one layer.(you can do this by saving the sig as a .png or just open the layers window [ctrl+L] and merge every layer down.
*Step 2: From the Layers window, select the only layer, and click "Duplicate Layer" Twice, so that you now have 3 layers.
*Step 3: From there, select the arrow across from "Layers", go to "Layers Menu", and click "New Layer". Make the new layer white.
*Step 4: Still keeping the Layers Window open, select the newest layer, and click "Duplicate Layer" twice, Like you did with the original image.
*Step 5: Using the 'Drag n' Drop' Technique, move the white layers to where they alternate with the background duplicates. The bottom Layer should be the Background image, the second should be the first white layer, etc.
*Step 6: Select the top White Layer, go to the image window, and select Filters/Noise/Scatter RGB.
*Step 7: In The window that came up, change the values to: Red- .20 Blue- .20 Green- .20 Then, Deselect "Independent noise" and select "Correlated Noise". The Alpha value should stay at 0.00. Click ok.
*Step 8: In The layers window, highlight the second White layer, and hit [ctrl+F] to repeat the Scatter RGB effect. Do this with the remaining White layer as well.
*Step 9: Select the top white layer again. In the Image window, select Filters/Blur/Motion Blur. Change the Blur Type to 'Linear'. Change the Length and Angle Values to 5 and 45 respectively. Click ok.
*Step 10: Back in the layers Window, select each white layer and hit [ctrl+F] to repeat the motion blur using the same parameters.
*Step 11: Now, in the Layers window, change the 'mode' of each white layer to "multiply". Then, select each white layer and, using the Layers menu, merge each one down onto the background image under it.
*Step 12: Now you should have 3 Layers: Background, Background copy, and Background copy #1. Got it? good. you're doing it right. Now, in the actual Image window, select Filters/Animation/ Optimize (for GIF).
A New image window should appear. Select Filters/Animation/Playback to preview the effect.
*Step 13: Now, Close out of the original image window, but You know the one that opened when you optimized the image for Gif? Keep that one. Go to File/save as and save at as whatever you like (Be sure to put the .gif suffix on the filename), making sure to Select the "Save as Animation" option in the Export File window that will open.
Host it via your favorite image hosting site and enjoy!
If you have any questions, drop me a PM or catch me on IRC.