I just wanna to open e-mail messages on my Thunderbird and see attached images without the need for manually opening attachments, like a lot of webmails, that show at least their thumbnails...
Thunderbird already does this in default configuration. If it does not work for you, make sure that the image is really attached to the mail and not just linked in a HTML mail. Automatically fetching (and displaying) linked images would be a privacy threat, as one could see if and when you read a mail by looking at the server's log file. You can set exceptions based on sender addresses however.
Note: Thunderbird 12 is outdated and contains known security vulnerabilities. Update to current version.