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 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.