InfoPath has limitations when creating direct connections to a database:
http://office.microsoft.com/en-us/infopath/HP100950831033.aspx
Quoting the article (I've underlined memo):
None of the data fields in the main data source of the form store a large binary data type InfoPath will disable the submit data connection if the query includes fields that can store a large binary data type, such as pictures, images, OLE objects, file attachments, the Office Access memo data type, or the SQL Text data type.
I think your best bet is to communicate to the database via a webservice. Although this requires extra work (and you need an IIS server running ASP.NET) you are much more flexible on what you can build.
QDabra's DBXL makes connecting your forms to databases easier as the webservice is already provided to you (http://www.infopathdev.com/files/folders/dbxldownload/entry19957.aspx).
David Airapetyan / Software Services ArchitectQdabra® Software / Streamline data gathering to turn process into knowledge