There is one recurring theme as I work my way around various websites, handle support issues, or work with new clients on importing old websites to a new framework. It relates to images. To ensure your website runs quickly, and does not waste bandwidth you need to optimise your images before they are placed on the website.
Over the years I have offered various guidelines on sizes, it often depends on the size of your template. But the core guidance remains the same. Do not upload an image to your website without scaling it down first.
Images are getting larger and larger with high resolution sensors. Your camera may well be able to produce an image which is 5000 pixels wide. However your website is only 1000 pixels wide, and the available space for an image in some cases may only be 500px wide.
Please check out this video which I put on the site a couple of years ago. It talks about free image editing software called IrfanView and how to use it.