1024 x 1024 isn't 16:9, either. Either the plasma screen doesn't use square pixels, which is a possibility but makes creating a presentation quite difficult, or its native resolution isn't 1024x1024.
First thing I'd do is double-check the native resolution. If it really is 1024x1024, then images presented on it will tend to look distorted, because the pixels aren't square.
If the resolution ISN'T 1024x1024, then you'll want to do exactly what you did--make the presentation 1024x576. That's an unusual pixel dimension, and some graphics cards might not support it, but it might be your best bet.