Since the iMac uses a laptop graphics GPU, it's not on a card as you know it (I'ts a really small card - daughtercard size), it's more of a module. So no there is no fan on either of the graphic chip modules. There is no space for such a thing. The iMac cooling system is probably responsible for keeping the graphics GPU cool. Maybe there's a heatpipe or heatsink specifically for the module.
Here is a link of what it would look similar to: http://www.hothardware.com/articleim...ll_module1.jpg
more:
http://www.hothardware.com/viewartic...?articleid=524
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