While metadata and header tags have historically held high importance for SEO, images should be up the priority list as they hold more significance and have a higher presence in universal search than every before.
Done right, optimizing images can be fairly and easy, and significantly affect site rankings too.
- image alt tags: Every image on a site should have an alt tag that is short and descriptive.
- filenames: Before uploading an image, pick a descriptive filename (different from the alt tag).
- Anchor text: Linking to images with text can play a large role in how they rank for keywords.
- Surrounding content: A page that contains at least 200 words of quality, relevant content surrounding an image helps define it and is a key component in getting it ranked.