Since search engines cannot understand images yet they use the “alt” tag (<alt>) descriptions to categorize any images that they might find.  When you hover over an image, or click on the properties of a photo you will see the “alt” description pop-up.  Empty or missing alt tags are a wasted gift.  Use them, and use them wisely.

• Use keywords in the description.
• Shorter is more powerful, but long descriptions are OK.
• Sign up for Google Webmaster Tools and make sure that you are allowing Google to index your images on your website so they appear in the search engines results.

See Also: keywords

A bit more info: here, and here.