How can I add an image overlay with a relative size?

When adding image overlays, we offer the following options for setting the overlay size:

1. Specify the overlay size in absolute pixels.
For example, 100px height:

overlay1

2. Specify the overlay in percents of the original overlay size.
For example, 80% of the original overlay's height:

overlay2

3. Specify the overlay size in percentage of the containing image.
For example, 80% of the containing image's height (200px in this case)

overlay3