Well... if you had Hi Speed Sync selected, then the flash should have worked although it's not true Shutter sync as we know it (Jim!)
Quick down and dirty explanation....
Your camera has a focal plane shutter, which is the only sort of shutter an SLR can have. They had to take it out of the lens so that you could see through the lens all the time, so they moved it right to the back of the camera where it sits right in front of the sensor.
It has two metal curtains. When you press the shutter button, the first curtain travels upwards, exposing the sensor for the desired time, then the second curtain follows it up, covering the sensor again. As the shutter resets, the second curtain winds back down dragging the first curtain with it, ready for your next shot.
The downside of a focal plane shutter is that the flash has to fire when the first curtain has gone up and before the second one starts it's travel in order for the whole scene to be illuminated by the flash.
With your 300D the fastest shutter speed at which this can happen is either 1/200 or 1/250..(I'm not sure) To achieve faster shutter speeds than that, what happens is that the first curtain starts it's travel, but before it's completely uncovered the sensor, the second curtain begins it's travel too. The effect is one of a moving slit (the gap betwen the curtains). The faster the shutter speed, the narrower the gap between the two curtains. In other words 1/8000 of a sec is achieved by the sensor being exposed for that duration
incrementally.
Obviously, the flash cannot sync with this moving slit, it's duration (the flash) is far too short, so when it fired you would simply have a narrow part of your scene exposed ...representing the gap between the two curtains.
Hi Speed Sync is a modern marvel, and not really sync in the true sense. You can however use it right up to the max shutter speed of your camera. What it does is simply emit a string of continuous pulsed flashes throughout the duration of that slit moving across the sensor, so that the whole scene is exposed by the flash. It's fairly limited to large apertures and close distances to be effective, as obviously, smaller apertures and greater distances, make a demand on the flashgun it will struggle to keep up with.
You shouldn't really have needed it for your Paula shot as your normal flash sync speed or even one lower would have been fine.
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