You can use the new webp format for your image. Do not forget to set a fallback for browsers that do not supports this format yet.
If you specify only an extension as a fallback, only the extension will be replaced. It is useful if your image is at the same location and have the same filename. You can also specify a full URL as a fallback.
You will see the same pictures twice in browsers that do support webp format. The second image will be different in browsers that do not support it.