If you were using cms.file document type, you could use File import tool, allowing you to upload images (or other kind of files) and create corresponding documents automatically. However, it works for cms.file document type only.
In your case, I would recommend you to use our API and ensure it code. We have examples how to create new document of given document type, and also how to create document with attachments. You can find API examples on http://devnet.kentico.com/docs/devguide/api_overview.htm
, sections Managing documents
and Managing attachments and files
. Thank you.