The images you upload to your website should be no bigger than they need to be. That means no bigger in width and height, and no bigger in file size. Almost every site could do better in this respect. If your image sizes are optimised in this way, your site loads faster. If your site loads faster, it makes users and search engines happier. You get more business, and everyone wins.
From big to small: 5 free image compression tools reviewed on Search Engine Land is a great place to start doing the right thing. Then in future, before uploading an image, (a) you’ll have made sure its dimensions are no more than the largest size it’ll ever be shown; and (b) you’ll have used just the right amount of image compression to balance sharpness and file size.