This feature is based on a bookmark named "CoverImage." All source and Manual target templates provided by Doc-To-Help have this bookmark, and it is placed by default on the cover image at the beginning of the template. However, if you specify a different image in the Help Targets dialog, it will replace the default cover image that is embedded in the target template.
To define a new location for the cover image, you can edit the template (see Editing a Template), inserting a bookmark named "CoverImage" somewhere before the "ChapterHeading" bookmark (if it exists). Otherwise, the image will be inserted at the beginning of the document.