When holding a camera at arm's length in order to take an oh-so-artistic 'Myspace Picture', the Myspace Arm is the area of your arm that gets in the way, and forms a fuzzy, blurred splash on one side of the image.
Many a 'hawt pikturre' has been jaded by the Myspace Arm.
"That picture of Chris and Lauren at the Acer gig would have been pretty cool were it not for the Myspace Arm in the bottom left."
The arm used (usually the left held out as far away as possible) to get a well angled picture that makes you look pretty even if you are not. The picture is then put on myspace or facebook so people can comment about how good you look.
I have a good myspace arm because I have long arms and can take pictures with my left hand.