When you goto the Alternative Rows theme bit enter this in the Extra CSS Attributes box:
For those who don't understand that line, basically it just tells the background image to repeat on ONLY the 'X' axis (horizontally / across / left to right) and not up or down.
background-repeat: repeat-x;
That'll sort it out. 
For those who don't understand that line, basically it just tells the background image to repeat on ONLY the 'X' axis (horizontally / across / left to right) and not up or down.
