Do a Google search, gather a few promising blog posts, and test some of the plugins. We don't know your use case.

EWWW Image Optimizer and WP Smush are two that have worked for me.

Yoast seo is the best. you have to do follow it guide in list. it is very easy

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YOAST SEO is by far the best plugin, they even do your Keyword research for you!

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You don't need a plugin.

Try Yoast seo of All in one seo plugins..

It's obvious that none of you recommending Yoast read the question.

The OP is asking about image optimisation.

My suggestion is you take a look at https://wordpress.org/plugins/wp-smushit/

As Yukon said above, you really should be trying to reduce your image file sizes BEFORE you upload.

i mentioned bellow the best wordpress plugins for image optimization...
1.WP Smush.it
2.Optimus
3.EWWW Image Optimizer
4.CW Image Optimizer
5.Hammy
6.SEO friendly Image

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