Providing descriptive alternate text for all images, via the ALT tag or detailed adjacent informational text (or captions), is essential for good Web Content Accessibility. It has been my experience that not providing meaningful alternative text for images is the most frequent Accessibility error committed by Web Authors - fortunately one that is easily corrected. I think Appropriate Use of Alternative Text by WebAIM is an excellent reference and guide for complying with this essential WCA requirement.
I think your comments should be descriptive, You can also see here about more on ALT tags http://www.mattcutts.com/blog/matt-video-alt-attributes-for-images/