Alexa offers a couple of easy methods to update your site thumbnail image.

Method One:
(1) Enter your domain in the web search field at http://info.alexa.com/
(2) Scroll down to bottom of your site info page and select the "Correct errors and omissions in this listing" link.
(3) Select "Update Thumbnail Image" link.
(4) Alexa will then update the site thumbnail image file within 48 hours.

Method Two:
According to Alexa's FAQ you can request their crawler to refresh your site image.

I've used both methods on several sites and actually prefer the first one. I hope that info helps.