Assuming you plan to change the background every 10 seconds, create a form level integer variable and initialise it to 18 (6 changes * 3 minutes) Put a timer on the form and set its interval property to 10000 (10 * 1000 = 10 seconds) In the Timer Tick event handler, check the form level variable if it is 0 then disable the timer, otherwise, decrement the variable and change the background.
Hope this helps.