Importance: High

Type: Relevancy

Properly formatted / coded images can help search engines determine relevancy.

Example:

<img src=”http://yourdomain.com/images/big-green-apple.png” alt=”Big green apple” width=”” height=”” />

The images file names should be descriptive and avoid using more than 3 dashes, with each dash containing a keyword.

Use dashes and not underscores http://www.mattcutts.com/blog/dashes-vs-underscores/

The alt attribute should be descriptive.

Reference: http://searchengineland.com/9-seo-quirks-you-should-be-aware-of-146465