You can customize the Alt tags on the images on your site to help with SEO and Section 508 compliance when needed. (Note that images that are presentational only, and not informational, don't need Alt tags for Section 508 compliance.)
To customize image Alt tags, simply put the Alt text into the proper field below the image. Experts suggest that you keep it under 125 characters due to Jaws reader restrictions.
Note that while search engines and screen readers will see the Alt tag, the general public will only see the caption in their browser.
Learn more about image Alt tags in this great article by A List Apart.