Pixel Dimensions the elephant on the page.
Image Optimisation
Image Optimisation is a widely accepted method of reducing the file size of an image without affecting its resolution greatly.
SEO is not Rocket Science it’s common-sense. Be proactive — get it right from the get-go. Wordpress allows you to use some great SEO plugins — like Yoast. You’ll be on the front-foot straight-away.
Should I use an Image Optimising Plugin? It all depends on time and money is the short answer.
Getting Image Optimisation In WordPress Right for SEO is vitally important