Transparent overlay image to introduce web site features/elements

The transparent overlay image to introduce web site features/elements is an interesting UI Pattern to do more with less - a visual explanation is better than reams of notes.

Don't know if Gmail was the first popular website to use this idea but I now see a lot of other websites use this kind of an overlay to visually explain new UI elements or features that are added.

Gmail used this overlay idea earlier to show Gmail keyboard shortcuts with the "?" keyboard shortcut.