How to Drastically Speed Up Your Website? Here's How I Did it.
Step 4: Re-upload All Your Website Images
Like I've said, to achieve the best result, you have to re-upload your images to your site and delete the old ones.
I know this can be very time-consuming and troublesome if you already have an established website with quite a number of images.
So, if you feel your site speed is decent and it's not very slow, you can get away with just cropping and scaling your images directly in your WP Dashboard.
However, if you feel your website is very slow, I would recommend you to take some time and do all the steps in this training.
That's what I did with my website and the site speed improves a lot.
(P.S While doing all these resizing, compressing and re-uploading, you can still have an image optimization plugin activated)