Image optimization is complicated and a multi-tiered process that takes months of study and practice.
Just kidding. It’s easy.
First, make sure you’re images aren’t huge. Most digital devices take pictures in high resolutions and most stock photos come that way. Always size your images to the exact size they’re going to be online. If you’re using relative widths with a responsive design (eg 100%, 90%,) then size it to the max width you’re using and consider using media queries to scale down the image if possible for mobile devices and tablets.
Second throw your images through an optimizer to reduce their overall “weight”. This won’t affect image quality most often and will reduce the KB of your image by up to 95%. So a 100KB image becomes more like 5KB. This is significant. The best image optimizer (free) on the market it RIOT. http://luci.criosweb.ro/riot/ RIOT is easy to use and does a great job on jpegs … not so much on transparent png’s. Still it is very useful and best of all, free.
Ultimately, that is it. Size your images and run them through an optimizer …. aaaaaand…. you’re done. Simple. Livewire.