My first attempt at a lightsig turned out nicely! What you actually see here is the light of my camera's 4x3cm LCD screen moving through the air, captured in a 10 second exposure by Martijn's camera.
By the way, I could have sworn this sort of thing was commonly referred to as a lightsig, but the Google non-results seem to indicate otherwise.
To make the dot above the I, you could simply put your hand on the LCD.