Image publishing checklist

1. Don't embed important text inside images
2. Describe your image. Give a good description. Beware of long "alt" attibute values. Or google rank you as a spam because of "keyword stuffing".
3. License your images. Add a small "Terms of Use". You will like Creative Commons.
4. Generate your images different size formats for your needs.
5. Specify your image dimensions in html.

Read more http://support.google.com/webmasters/bin/answer.py?hl=en&answer=114016

future reading https://www.magic-picture.com/5-best-ways-to-optimize-images-for-seo/