We are setting up a portal and adding group thumbnails. We have added an incorrect thumbnail for a group and want to reset the thumbnail to empty, so it just shows up as a box with the groups first initial. (The reason is we do not have this group's thumbnail ready yet). There only seems a mechanism to provide a new thumbnail, not delete the existing one.
I don't believe there is a way to "delete" and remove a thumbnail without providing a replacement image. As a workaround, I would just upload a "blank" white image as a placeholder for now - until you obtain the appropriate thumbnail image.
Hope this helps,