As a short image files size is important when considering the loading speed of webpages, it exists free tools to reduce .png and .jpg files without loosing quality. Here are 3 of them: Tinypng, Kraken and Smush.
Tinypng
Tinypng is a free online image file compressor.
Simply drag your original image file (.png or .jpg), wait a few seconds and you can download the same image file with a reduced size.
WordPress plugins
WordPress blogs platform benefits from two free plugins to deal with image files size: Kraken and Smush.