A seam is a line formed by sewing together pieces of cloth. A seamless image or some may call it tileable background, can be repeated without any seam being visible because the edges of the image blend well.
This step-by-step tutorial will show you how to create a seamless background. Such images are good for use as backgrounds or textures in 2D scrolling games and 3D rendering.
Start the GIMP, here I’m using GIMP 2.6.11.
Create a new image (Ctrl+N key). Put the size you want on the Width and Height fields. You should use the size of power of two if the image will be used as a 3D game texture, because for optimization reason and some game engines will require you to use the size of power of two. That means the sizes are limited to: 16, 32, 64, 128, 256, 512, 1024, 2048 pixels, … and so on in both width and height.
Do some painting using Paintbrush Tool (P key). Try to use different brush shapes, sizes, colors and opacity.
Use the Make Seamless filter and GIMP will magically make the image become a tileable texture. You can find it on the “Main Menu” > “Filters” > “Map” > “Make Seamless”.
Step 5 (Finish)
The image is now a seamless texture.