how do I theme rounded corners in Panels 7.x-3.3
The functionality you refer to predates common browser support for rounded corners, when images was the only reasonable solution.

Today, I would most likely no use it, but instead implement it with the modern CSS approach.

If you really want to modify the existing functionality though, it's called a Style. You can see this question for details about implementing ctools plugins: What are Ctools plugin (content type, access, etc), and how does one create them?