They are jpg because the game itself uses jpg. They are ripped directly from the game. Your png re-encodes in this case actually have worse quality and are 5 or more times larger.
You can find them by typing [[File:<ship_type_code> <ship_name> and auto-complete will find them all for you. For example, [[File:DD Kawakaze will bring up all of Kawakaze's assets. The ones with "Battle" in the file name are the battle cards you want. "Full" is the CG, and "Card" is the normal card. There are also the damaged variants.