There's a great alternative to PNG called ZorroSVG that helps to optimize size of transparent images making lightweight JPG inside SVG masks. Results of the compression are great — up to 80% from the original size.
More details here — http://quasimondo.com/ZorroSVG/
The developer says that this technology based on SVG 1.1 so it supports all the current versions of web browsers out of the box.
The only thing... It doesn't work with Webflow.
I tried to add the example image to the page but the only thing I see instead is the blank picture — http://pagediscount-beta.webflow.com/untitled
Guys, maybe you could take a look on it?