Image size is also a important factor to speed up a site.if you really want to increase your ranking and speed then just try to findout the real problem behind the low speed.
There are lots of tools on the internet by useing them you can identify and calculate your problem.use cloud server for your images.after using cloud server you can reduce loading time of images.after then check your themes CSS that it is supporting your website or not.
@Soapmarine Most of the website speed decreasing problem , Because of the Quality Images . Optimize your website image as much possible . There is some websites on the internet , Which allow you to optimize your images without getting quality loss of your images .