(Did you know?) that jpeg screenshot can be compressed selectively?
That's usually not THAT useful, unless your beautiful screenshot is about to hit a single file upload limit and you don't want to sacrifice image quality.
Example: Aliens: Colonial Marines
regular 2560p jpeg from png, 85% quality(click to fullscreen): (850kB)
and selectively compressed: alien queen and some close buildings and [email protected]% quality, rest of image @75% (690kB)