ImageView, Labeled, Region and Tooltip can reference images. There is 2 places where the img can appear: Author CSS or Inline CSS.

The image can be retrieved when CSS is applied, doing a check each time CSS rule changes would be nice (tracking only file that are actually in use) but would be costly. We are in the critical perf path there.
A possibility is to do it on the CSS source (style property or file)
- When the style property is updated.
- When CSS file are loaded/updated.
The parsed StyleSheet structure should contain the path to Image.