After further investigation, unfortunately, Alexa doesn't allow anyone to upload a thumbnail image but themselves. But, they can do it for you using a crawler.
Click here for a link to get Alexa to upload a thumbnail photo of your blog. Then, click on the link where it says "If you would like to request that we send our crawler to refresh or capture an image for your site, you can submit the URL with this form:" http://thumbnails.alexa.com/update_thumbnail
Type in the URL of your blog and wait for at least 48 hours. I've just done mine and hopefully it works but at least, worth a try!
This Post was written by Mae from riceandnoodles