Haveing created a links page using this service & noting that after over a week some sites still have no image and others have 'blank' images I would suggest that anyone considering this makes sure they have the option to use alternative images, as they cannot rely on those from here.

I know the 'blank' issue is blamed on the sites servers or site design, but I`ve seen the issue on nearly 10% of sites. Regardless of the cause its still an issue.

I was able to find the answer here, /manage/updatethumbshot.aspx.

Thumbshots robot automatically determines whether your site should be updated more or less frequently depending on the activity of your site. Most sites hardly change or the change is too subtle for the eye to detect (e.g. the change of a date on a page is not visible to the eye). With time and experience, the robot will adjust to optimally match your site's activity. You should be patient and let the robot do its job learning to adapt to your site.

I can also use this page to submit my sites to be updated.