It seems that either a "BB code" mor a URL will make a photo appear in your Comment.
1. Edit the entry using the icon like a pencil.
2. Copy a BB code or a URL from some photo that already exists in the NGA site. For example, add your photo to the plant database, then copy the BB code or URL from that photo.
3. scroll all the way to the bottom of the "edit" dialog. Paste the URL or BB code into the comment. You can have other text in the comment, as well.
4. Save Your Changes.
In the Plant Database, while viewing a single photo, here are the URL and BB code locations:
