Creating and using seamless texture sets
There has been a fair number of posts on creating seamless textures for use in maps, the largest one here.
Extending on that topic, here is a technique to create seamless texture sets, or multiple tiles that are all seamless with one another. I first read about the technique in the dundjinni forums, as that program supports tile randomization.
The original site is not available: http://www.vsf.cape.com/~jdale/dundjinni/tut_tiles.htm
but there is still a cache in the wayback machine: http://web.archive.org/web/200701082.../tut_tiles.htm
As I don't know how long that will be available, I will walk through the process here.
I am starting with an image from CGTextures that I have scaled down:
I want 200x200 px tiles so I set the rectangular selection to be fixed 200x200:
And copy and paste a selection to a new image:
Which is made seamless using your preferred technique:
This will be the base image that all other tiles will be made from. Duplicate the layer and make a squiggly feathered selection in the middle: