This problem you are seeing with only grabbing a white frame was a bug in the CMU demo app. There is an update posted
here that should fix the problem. You were indeed, correct about the USB converter, though it's not the speed that caused the problem, but the jitter, or burstiness of the speed that defeated the code in the get picture VI. That problem shouldn't be noticable on a standard serial port.
Best of luck!
-Joe