1. 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.

2. Next, select the new file, and apply Edit/Fill. Within the Fill dialog box, select Use, Pattern; Opacity, 100%; and Mode, Normal The resulting image will be a seamless pattern tile, as seen below.

Extra Credit: The Clouds filter is a great place to start explorations into creating different textures using the filters that come with Photoshop. Try starting with the Clouds filter, duplicate the image by selecting Image/Duplicate, then apply some of the Artistic, Noise or Texture filters. Too tame? Try applying Image/Adjust/Invert (Control+I on Windows, Command+I on the Mac) to create a dramatic change in the contrast and color in your image.

Above you can see the seamless cloud background with nine variations. From the upper left: the original image, Artistic/Cutout filter applied, Artistic/Paint Daubs filter applied, Artistic/Plastic Wrap filter applied, Artistic/Rough Pastels filter applied. Second Row: Artistic/Sponge filter applied, Artistic/Underpainting filter applied, Artistic/Watercolor filter applied, Distort/Diffuse Glow filter applied, and Image/Adjust/Invert applied.