This will be added in 1.5.1.
The effect has the following constraints:
- The image has to have the exact same source URL. If the image is wrapped with a different link (say an unscaled version of itself), it will open in the browser.
- The image has to be cached. To provide the effect instantly, I get the cached response from the URL and try to convert it to an image. If that fails, the image will open in the browser.
You can try this change when it's built by becoming a beta tester.