Opinion please regarding image sizes and type

what in your opinion is "the best" way to have your images to be useful to be loaded in titles
As a practical example say country flags , depending on title they might be resized to often pretty small

do you prefer png or jpg (settings)
do you have them on file per size needed or does one prefer to reduce the size on the fly
Do you have them in different quality based on frame size to be used in
other things to consider

Thanks