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Check the case of your file names. The files "MyPic.png", "Mypic.png" and "mypic.png" are all the same when running on the simulator, but different when running on the device.
 
.png's problem

once,i also had the same problem,png can show up in iPhone Simulator,but it can't show up in device,because that png has some elements which the device can't identify
 
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