Since the URLs provided have been for Photoshop only, here is some info for GIMP users:
GIMP has special filters useful for mapping under
Filters > Map. For example,
Filters > Map > Tile makes images tile seamlessly by applying a ghosting effect. Some Filters which generate stuff, like
Filters > Render > Clouds > Solid Noise or those under
Filters > Render > Pattern do have a "Tilable" option, which will take care that the result tiles seamlessly automatically.
To make your image tilable by hand, you can follow the same steps as in the guides above, look at the
Tileable Textures tutorial or the more sophisticated
Making Tileable & Seamless Textures tutorial for reference.