I found this cool plugin that will be a big help for custom textures . I had a texture for a Motocross Madness track for the offtrack texture that kinda looked rocky . I really liked how it looked but the seams were real bad . I had plans to go back and try to fix it up somewhat .
When I found this I loaded it up and tried it since I could load this in the MCM2 editor and view the results in a game rendered situation quickly . It worked like a charm . Seams were gone . Check it out , I think you will like it . Here is the link .
http://www.geocities.com/murakuma/mmplugins.html
Scroll to the bottom of the page .
Edit:
Here are 2 results shots . I chopped a piece of a pic of a tree lined cliff , sized it and (480 x 480 in this case) and loaded it into the MCM2 editor . Chances of this tiling as is are slim to none .
Then i opened it in PSP7 and ran this filter on it . Here is the second shot .
Looks like you can make nice tiling textures from prit' near any pic .