Hero images are quite large / slow to load

[thisandagain]

On the landing page, the image hero-unit@2x.png is about 737KB. I noticed this as every time I load teach.mozilla.org I notice a long loading delay (even on Mozilla Wifi). Doing a quick test, exporting as JPG at 60% quality (no noticeable degradation in quality) cuts that in half. Other hero images also appear to be about half a megabyte or more.

[hannahkane]

I think this is a dupe of #790.