Sure, you're welcome.
If you want to update only documents and related images I think you should be able to just pull out the data of the CONTENT_tablename
table where the document data is stored. However sometimes those tables can have links to other documents/custom tables or files so these connection might not work, especially if they are bounded with ID's instead of GUID's which is a common problem I see. The ID is database based and can (will) change when exporting/importing the data. On the other hand its always better to use GUID as they will not change and it servers as an unique identifier. However it really depends on how your documents look. Also if you are storing files/images in file system you will have to include those files too and not just physical files, but their reference in database.
Like I said in the previous post, it is best if you could use our API to create documents, you can see how to create them in these examples -> http://devnet.kentico.com/docs/devguide/documents_api_examples_creating_documents.htm
It would be easy if it would be possible to export just some specific document but unfortunately this is not possible.
Let us know if you have any other question.