Jix comes with a bonus app, Tilex.
This turns an image into something that will tile without joins.
You open an image, crop it and mirror it.
If you like it, save it and try it on your computer desktop.
This doesn't sound much, but I've found it dangerously addictive.
Some things make unexpectedly beautiful tiles: sections of clothing, buildings and plant life pleased me particularly.
When you're cropping, look at what's near the edge of the crop.
Most of all, trial and error will be rewarded, so if it's not good the first time, use the undo button.
I have put up a gallery of tiles - to get the full effect, try them on your desktop.
If you're on Windows, installer sets Tilex up for first-time users as a tool.
If that hasn't happened, you can set it up yourself: on the Tools menu, choose Configure tools... and put this in the command box:
javaw -classpath "C:\Program Files\Jix\jix.jar" Tilex "%f"
If you're not on Windows, setting Tilex up for yourself is very similar.
Try something like this in the command box (fixing the path as necessary):
java -classpath "/home/myaccount/bin/jix.jar" Tilex "%f"