There are a number of reasons why the sharing url link does not show the thumbnail image on Facebook. Most frequently because the first image found on a page doesn't match the minimal image requirements (The image must be at least 50px by 50px and have a maximum aspect ratio of 3:1. Supports PNG, JPEG and GIF formats.).
To get an insight into the problem you can use the debug website on Facebook: http://developers.facebook.com/tools/debug. If you use the CPTCHA Image module it might be the cause of the problem, because it produces images that don't fit the minimal requirements.
If so you can define the image to be used yourself using the following meta tag. Since you are sing Drupal you can use the Metatag submodule Metatag: Open Graph to do so.
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