This instructable will help you to make images seamless in only the dimension of your choice.
Step 3: Paste it in a new transparent layer
Make sure you paste at the right 1/3rd of the image.
Step 5: Paste it in a new transparent layer
Make sure you paste it at the left 1/3rd of the image.
Step 7: Blending the masks. Step1
(make sure the 2 colors are black and white)
Step 8: Blending the masks. Step2
Step 9: Blending the masks. Step3
Step 11: Testing & Finish
To test, use the "tile" plug-in. It is at various places in different versions. So i wont instruct on how to use it.
These instructions were for making an image seamless horizontally. You can adapt them to make images seamless vertically only too.