Thumbnail not updating

In manage galleries, I go into the gallery, and click on edit thumb next to the picture. i change the thumb and click update, then save changes. it shows my new thumbnail in that page, but when i go to my website to view the gallery, the thumbnail is still the same. i’ve tried clearing my browser cache. the only thing that seems to work is to go to other options – misc – clear cache, but i can’t have my client clearing the cache every time he wants to update a thumbnail. any ideas how to get around this?

http://wordpress.org/plugins/nextgen-gallery/