In photoshop what you can do is... Select from the menu... Filter/Other/Offset... When the little offset window pops up you put in half the image pixel size in both the 'Horizontal and Virtical' text boxes and make sure 'Wrap Around' is selected at the bottom of the filter window. ie, If you image is 200 wide X 100 high you would put 100 in the horizontal text box and 50 in the virtical text box. This basically shifts the image around the screen so that the left hand and top sides move into the centre of the image and wraps the right hand side and bottom wrap around to meet them in the centre of the image. Then you have to play around with the clone tool particularly to fix the middle parts up. There are seemles texture makers out there but this is just one way of doing it :)