Thanks for testing the Editor!
I'll go through your bug reports and feature requests in detail after the weekend.
- How are the rounded corners and dropshadow made? Are these still the "outlines" from the "graphics" folder?
Yes they are. Your new outline is stored as custom.png.
- Would it be possible to add custom graphics?
When you use the Editor you always create a custom outline PNG. To edit design elements not covered by the Editor, you'll have to use CSS.
- What is the "editor.config.js" ?
It is the configuration settings for the Highslide Editor. For now you can't use it for anything, but in the future I'll probably add a feature where you can load a config file into the Editor, for example when you want to upgrade or do minor changes to your setup.