To use Textpattern’s Article image feature (under Advanced Options on the content > write page), you need to do two things:
1. Place an image URL or ID in the Article image field. The ID is a single number, like 123 – don’t use an extension. If you want to refer to it by filename, you’ll need to use an absolute URL like /images/cat_pic.jpg or http://example.com/images/cat_pic.jpg. Don’t put HTML or Textpattern tags in the Article image field, they’re not needed and won’t work.
2. Make sure your article form includes the following tag:
<txp:article_image />
More detailed instructions are included in Textbook.