Um...if you have Photoshop you can apply the "Offset" filter and then enter the width and height as half of the picture and change the type to "wrap around" then just use the clone tool to smooth the visible seams, then reverse the offset, and you have a seamless texture. If you need more help on this I could reexplain it or you can look on the internet for help, thats how I found it out
Hope it was helpful,
Adoado