Hi Sergio,
There is nothing built into GDIPlusX to extract data from General fields.
I have spent a bit of time trying to decipher the General field data structure, but it is quite a pain. There are API calls that allow you to load General field data and then render it to a HDC, but nothing, that I can find, that is a straight forward way of generically retrieving the raw data.
I added a function to my Moxie Object FLL library a while back that can extract most of what I have seen people stuff into a General field; BMPs, JPGs, GIFs, Word docs, etc... but it is still a work in progress.
If you are interested, I can post the library and some sample code.