Actually i have installed a theme and that has custom post type with categories. I have created categories and searched in database that are being inserted into terms and taxonomy tables. And thumbnail is inserted into posts postmeta tables. but i failed to find any relation between category and thumbnail.

What i have understood wordpress db structure. The image is on url.

The post 3230 is just for understanding, there is no post no 3230. Can someone explain me. Thanks in advance.