Images must be properly resized using graphics software such as Adobe Photoshop.
Do not use HTML code to reduce the width and height of an image as it will be transferred over the internet in the original size no matter what reduction this is set for, which will slow the loading of the Web page. Viewers are forced to download larger files than they need to and browsers do not do a very good job of resizing such images. Also, resizing in the browser requires more time to render the page and uses more of the viewer's system resources and RAM.
Cropping your photos before adding them to a Web page will result in smaller file sizes and less download time.
Specify actual settings
What is advisable is to specify actual image width and height settings. If these are left out, the browser can not build the page until the image has finished loading which means that the visitor cannot read text around this image and, in turn, will give the visitor an impression of a slow loading page.
Images should not be animated, unless there is a special need (in which case you can submit a request for approval to OCP.