PNG or JPEG?
And in this battle, if we do not need transparency, PNG loses, so to speak, the "mid-market". Although, if you link the JPEG'u to ICC-profile, the difference in volume is negligible. My measurements quoted on - JPEG 100%. And at a lower quality (not very noticeable difference to the eye) comparison, unfortunately, is all the more sad.
general, the results are considerably dependent on the image itself: PNG compresses better gradients and plain areas, but for photographs gives better compression JPEG. Although, they say (very hard to believe), the large photographs PNG and then lead - checked just in the process of converting to the article screenshots (PNG vs JPEG-70% - winning half times).