W3C Validation Error

I got this error message through W3C’s HTML validator:

Line 592, Column 131: An img element must have an alt attribute, except under certain conditions. For details, consult guidance on providing text alternatives for images.

/wp-content/plugins/usernoise/images/ok.png” id=”thankyou-image”>

Should be an easy fix. Thanks!

http://wordpress.org/extend/plugins/usernoise/