Seroundtable has a great conversation on image optimization with suggestions from some pros including: 

• He says that you should enclose your images in divs (defined by a photo class).
• Image titles should be in h3s or larger.
• Images that you want search engines to know about should be 35-40% of the page size and should be on the top of the page.
• The image name is still important.
• Captions should be added to images.
• Never forget those ALT tags (but TITLE tags are not as important).
•  You can refer to the image from somewhere in your content to boost its relevance.
• Search engines are getting smarter at figuring out what is in images (face recognition, for example).

Visit Serountable to see the complete list.