TinyMCE buttons by content type
You need 2 text-formats.  Allow "Image" in tinyMCE profile of one text-format and not allow it for the other.

To set fixed text-format to each text-area field, you can use Better Formats module.  Download from http://drupal.org/project/better_formats and install it.

In "Managed Field" page of your text-area field, check "Filtered text (user selects text format)" and "Limit text format".  You will see all your defined text-formats with leading check-boxes.

Click the check-box of a text-format and save.  In your "Add Content" page, you will see your text-area is assigned with respective tinyMCE editor.