It's still relatively huge, and not widely supported without a polyfill, but FLIF [0] is a pretty neat format that tends to be smaller than the equivalent PNG, and can be converted to a lossy image "on the fly", and also supports alpha transparency.

The polyfill is still kind of slow, at least compared to native PNG rendering on the browser, but it does work, and hopefully some day it can be integrated into the standard.

[0] http://flif.info