There are tons of online image compressors out there, but my favorite one is ILOVEIMG. It allows you to compress up to 30 images at once, and has a pretty minimalistic no-BS UX, and is pretty fast for a free tool.
If you want to bulk compress more than 30 images, you’ll need to sign up for a paid account, which currently is 9 USD per month, and even that is a fair deal if you ask me.