AS: You mentioned wanting a different shape on which to paint. Perhaps you've already thought of this one... but the idea of a large keyhole popped into my head. It would provide a fairly significant area for the image... while giving the sense of looking into/onto something.
And if one wanted to make a connection between the image and the keyhole shape, there could be a being in the image who was holding or dragging a key behind them... or perhaps grasping at keys floating all around and/or just out of reach... as if to show they are locked in somewhere, while struggling to discover/reach the keys to their freedom/understanding.