How can add Thumbnail image in Drupal 7 site? I have set an image style as a thumbnail, but it doesn't render as my thumbnail style.  Instead, it is showing the original size of image.  How can I solve this problem.
You can edit the content type and configure in the manage display fields. Go to admin/structure/types/manage/article/display considering article is a content type.Replace 'article' with what every node type you have and change the image fields display options as shown in the screen shot.