You'd need to use javascript, OR an animated gif, OR a flash animation, depending on your preferences.
Animated gifs can be created in fireworks - you add frames, and adjust the lenght of time between images showing, export as animated gif.
Remember gifs are not good for photo type images with lots of gradients of colours - these need to be kept in jpeg form to look nice, so flash or javascript would be prefereable then.
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Sometimes, when my code just won't behave, I take it outside and make it listen to britney spears music, and when it comes back it's really well behaved. I wonder if it's suffering from post tramatic stress syndrome now..