This plugin is no longer necessary for most users because similar functionality has been integrated into b2evolution's core. For more information, please read: Image Sizes
The Thumbnail Plugin allows you to set a maximum image size for your blog. Every local stored image larger than this value will be automatically resized to fit this size. Creates automatically thumbnails from images bigger than a specified size. If a lightbox2 is installed it can create automattically a lightbox-link to the original image (works also without lightbox -> than it's simply a link).