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Author: Captain Jack
This demonstrates a fairly simple method of adding rain to an image using Paint Shop Pro 8. It will also probably work in other versions and software, although I've only tried it with PSP8.
Add a new raster layer to the base image: Blend mode normal, opacity 35%.
Flood fill the new layer with a solid foreground color of light gray (R: 192, G: 192, and B: 192).
Create a new image that is the same dimensions as the original picture in 16 million colors.
Flood fill the new image in solid white.
(Note: if, at the end, your rain seems too large, undo everything and try again, making this new image double the size in both dimensions, then scale it by 50% after step 3.)
Select Adjust - > Add/Remove Noise - > Add Noise: Random, 50%, Monochrome.
Select Adjust - > Blur - > Motion Blur: Angle of 220.0, strength of 30%.
Select Adjust - > Brightness and Contrast - > Threshold: 230.
In the original image, select Layers - > New Mask Layer - > From Image: Image created in step 2, Source luminance, Invert mask data.
Select Layers - > Merge - > Merge Down.
Click the visibility toggle (eye) in the layer palette for the background image to make it invisible.
Select Effects - > Edge Effects - > Enhance.
Click the visibility toggle on the background layer to make it visible again.
Select Layers - > Merge - > Merge all (flatten).
Save your modified image.
Delete the image created in step 2, as it's no longer needed.
Adjust the transparency in step 1 to make the rain less/more visible.
Adjust the color in step 1 to lighten/darken the rain.
Adjust the angle in step 3 to change the rain direction.
Adjust the threshold in step 3 to make the rain larger/smaller (lower values make less rain).
To make the rain look thicker and more three dimensional, repeat the process making the second rain layer more transparent (step 1) and smaller (step 3 changing the threshold value).
To make the rain look brighter, like rain from a small cloud with the sun shining, stop at step 5, and repeat the process with a new rain layer, making the new rain layer about 90% opaque and using white as the color (step 1). At step 5 the second time, use the move tool to carefully move the new rain layer one or two pixels up and to the left. Drag the new rain layer so that it's between the background and the first rain layer, then finish the image from step 6.
Happy rain-making! :)