I have searched Google for this and there has been alot of conversations, so a very common problem, but no resolution by the looks of it?
I've been using the NextGen gallery plugin for a while on many sites, but having installed a new version of WordPress, I have downloaded then added the newest version of NextGen 1.9.0, and the thumbnails simply won't load.
I've deleted, uninstalled, both the images, galleries and the plugin and nothing has worked. I also tried installing an older version of NextGen I had on my desktop, there just seems to be some problem between WordPress 3.3 and this usually so reliable plugin.
As an aside what would be an alternative gallery plugin which will allow me to have a scrolling thumbnail, link to a larger image above?