Where to apply the PNG?
For transparent design elements should use PNG-8 (rarely PNG-24, when the palette PNG-8 is not enough to save images without losses).
for photographs and complex graphics leader remains powerful features for JPEG image optimization. Although in some cases it may be sufficient and PNG-8 - as before, with GIF'om, daring experiments - the key to success. But for the really big IMAGES JPEG still loses.