ALT tags have little impact on SEO, but for sites in enormously competitive industries, every little bit counts.  ALT tags were originally created to help the visually impaired surf the Internet more easily.  There are software programs available that read aloud the information in the ALT Tags.  These days, those with a strong understanding of SEO use that as a small advantage by placing keywords into the ALT tag information.  Again, this has little impact on SEO, so don't count on ALT tags to make a significant difference to a site's ranking. 

General Guidelines 

• Search engines can't read images, but they can read ALT tags - make them descriptive.
• Should be separated into 2 parts:
  • Image description
  • Brief keyword/keyword phrase description
• Parts should be separated by a "space hyphen space" (" - ")
  • Example:  "Dog - Pet store offering dog food, pet toys, crates, and health supplies"