theme_image_style needs to work with image paths again.

theme_image still works with a path. People expect theme_image_style to work the same way. If it's better to use a URI, can we reverse-detect the URI based on a path, and the site settings, so that if a path is provided this function can still work?

This is a function that Drupal beginners will encounter early on, long before they should need to understand URIs or why they need them.

Image styles are awesome, we want to encourage people to use them, and to love them. But we shouldn't make it this hard to use.