In WordPress, thumbnail image is the minimized version of the main image. Sometimes thumbnail image is not proper re-sized,
unprofessional looks, stretched and blur image view. Are you looking for different thumbnail image of original image?
Zengo Custom Thumbnail Image plug-in helps to define separate image for thumbnail image and main image.
You can assign the proper designed version of the main image as thumbnail. Or you can assign the different image as thumbnail as per your requirement. Also there is facility to customize the thumbnail image size using WordPress default or custom plugin functionality.
For Support:
http://www.zengo-web-services.com
Limitation: Zengo Custom Thumbnail Image plug-in can manage only one Gallery.