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Combining images into as few files as possible using CSS sprites reduces the number of round-trips and delays in downloading other resources, reduces request overhead, and can reduce the total number of bytes downloaded by a web page.
Similar to JavaScript and CSS, downloading multiple images incurs additional round trips. A site that contains many images can combine them into fewer output files to reduce latency.
Specifying a width and height for all images allows for faster rendering by eliminating the need for unnecessary reflows and repaints.
When the browser lays out the page, it needs to be able to flow around replaceable elements such as images. It can begin to render a page even before images are downloaded, provided that it knows the dimensions to wrap non-replaceable elements around. If no dimensions are specified in the containing document, or if the dimensions specified don't match those of the actual images, the browser will require a reflow and repaint once the images are downloaded. To prevent reflows, specify the width and height of all images, either in the HTML <img> tag, or in CSS.
Properly sizing images can save many bytes of data.
Sometimes you may want to display the same image in various sizes, so you will serve a single image resource and use HTML or CSS in the containing page to scale it. For example, you may have a 10 x 10 thumbnail version of a larger 250 x 250 image, and rather than forcing the user to download two separate files, you use markup to resize the thumbnail version. This makes sense if the actual image size matches at least one — the largest — of the instances in the page, in this case 250 x 250 pixels. However, if you serve an image that is larger than the dimensions used in all of the markup instances, you are sending unnecessary bytes over the wire. You should use an image editor to scale images to match the largest size needed in your page, and make sure that you specify those dimensions in the page as well.
Specifying a character set in the HTTP response headers of your HTML documents allows the browser to begin parsing HTML and executing scripts immediately.
It's important to serve a resource from a unique URL, to eliminate duplicate download bytes and additional RTTs.