Image Optimization is the process of ensuring that images are doing more good than bad for SEO. This includes writing optimized alt text, but Image Optimization is often one of the fastest and easiest ways to improve load time. In general, images should be loaded at the exact pixel size in which they will be displayed, rather than a %. They should be the highest resolution needed for appropriate viewing, but not higher. They should be compressed, and saved in the appropriate image file type. They should also be adequately cached and lazy-loaded in some cases. There are many nuances to image optimization, but it is an important aspect of SEO.