JPEG is not a good choice for UI/typography screenshots. There are ~1MB of JPEGs at quality 50 on the page. By using PNG8 not only you can halve that size but also preserve original quality. Example:
[1] PNG24 199 144 bytes
[2] JPEG 94 699 bytes
[3] PNG8 47 155 bytes
[2] https://user-images.githubusercontent.com/3129436/47068393-d...
[3] https://user-images.githubusercontent.com/3129436/47068392-d...