- Don't embed important text inside images
- Describe your image. Give a good description. Beware of long "alt" attibute values. Or google rank you as a spam because of "keyword stuffing".
- Generate your images different size formats for your needs.
- Specify your image dimensions in html.
future reading https://www.magic-picture.com/5-best-ways-to-optimize-images-for-seo/