Stereograms consist of two parts: The depth-map and the tiled image. The tiled image is what you see at first. The depth-map is the hidden 3D image. This tutorial will show you how to create the tiled image without having to paint a single stroke! Made for Paint Shop Pro 5, but I guess similar results can be achieved with other good paint programs.
Create a new image
Create a new image 10 pixels width and 40 pixels high and with 16.7 million colors (24 bit).
Fill it with a medium grey color.
Add noise to the image ( image > noise > add... ). Set it to 100%.
Tile it
This is an important step. Select the whole image and copy it to the memory.
Now create a new image twice the size the original (20x80).
Paste the original image into the new image 4 times so that it precisely cover all of the new image ( highlight the new image and use edit > paste > as new selection ). You can now throw out the original image.
Resize
Resize the image ( image > resize... ). Resize it ten times larger ( 1000% ). Use resampling ( smart resize ).
Blur the image heavily ( image > blur > gaussian blur, radius 10 ).
Adjust the colors
Play around with the color filters. You could for example do this:
Change the hue, saturation, and lightness ( colors > adjust > hue/saturation/lightness. Set hue to 50, saturation to 100, and lightness to 0 ).
Then make it a little more colorful ( colors > adjust > brightness/contrast. Set brightness to 0 and contrast to 40 )
Add a little noise, 10% or so. That's because the tiled image in a stereogram has to be detailed to work well.
Crop it
When you are satisfied with the pattern, crop the image so only the middle part is left. If the image is 200 x 800 pixels then crop the edges away so there is an image left which is 100 x 400. The cropped part doesn't have to be exactly in the middle, but the image left must be exactly half the width and height of the original.
That's all
Now you have a nice tileable image for use in stereograms (or as background for webpages or whatever). The reason why you have to do the tile step and the crop step is that all the resizing and use of filters don't apply to the edges of the image.
Experiment with other sizes, filters, and effects. Instead of starting out with a plain grey color you can also use an image of yours or quickly make something colorful. There's a lot of possibilities!
Remember to let me know if you make some stereograms of your own!