This Photoshop tip takes advantage of the fact that the Clouds and Difference Clouds filters in Photoshop will create a seamless pattern any time they are applied to an image whose measurements are some multiple of 128 pixels.
Start with a new file, 128x128 pixels, in RGB Color. Select white as a foreground color and purple as a background color. The filter we'll be using uses foreground and background colors to create its effect. Apply Filter/Render/Clouds. You can check the image for seams in this image easily. Open a new file (or Control+N), set to a 300x300 pixel size. Select the original cloud image. Apply Select/All (or Control+A on Windows, Command+A on the Mac). Next, apply Edit/Define Pattern.