I assume you are talking about the Thumbnail for Joomla Content plugin.
From the instructions included in that –
<blockquote>
This plugin is used to generate Auto Thumbnail in Joomla Articles. This must be properly configured to make the best use out of it. Below are important note.
In the ‘ Integration Mode ‘ on the parameter list on the right:
- ”Auto” mode: All joomla article will have this feature enable. To turn off plugin for specific article, use the {jathumbnail off} in that article.
- ”Manual” mode: You must you [jathumbnail] somewhere in the joomla content to enable this plugin for that article.
- ”Disable” mode: Disable the Thumbnail generation (DO NOT use the Disable feature of Joomla if you have used the tag {jathumbnail} or {jathumbnail off} already. It will be parsed as normal text.
The Manual mode is recommended for site with less images, the Auto mode is for site with massive volumn of images
Note: Allow off some specified images for an article
we use tag:
{jathumbnail off images=”images/stories/demo/world/rs-3.jpg,images/stories/demo/world/rs-2.jpg”}
</blockquote>
If that is not what you are talking about please post some screenshots to avoid confusion.