Users on slow connections will see the ALT text until the image downloads.
If the image fails to load in the user’s browser, the ALT text appears.
People using text browsers or browsers with images turned off will see ALT text instead.
Alt tags also make your site more accessible to visually impaired people using text readers. Even if your web site is content rich, the alt tags allow you to reinforce what is highly important, the key terms, within the content. People with disabilities visiting your site should not be subjected to an hour of the same keywords being repeated to them over and over again via a screen reader.
Be sure to use the keyword phrases that you also used in the copy of your page, title tag, Meta description, and other tags.
Do not try to stuff all your keywords into the “alt” attribute. It is recommended using no more than 2-3 per image.
Describe the image – do not just list keywords.
ALT tags are currently considered by all 4 major engines in their page rankings
Google would only consider Alt Tags if it was in a link and therefore considered anchor text.
The same rules apply to alt tags as to all other content – be sure it is natural, otherwise it may hurt your rankings instead of helping it.