First thing, please do not start two threads in a very short time span for the same problem.
The problem I ran into was that fpdf's image method expects a file with an extension for supported images. This is why it is a problem to pass it a binary data from the db. A workaround could be to write the binary data into a temporary image file, pass the path to the file to the image method and then remove the file but this seems a bit awkward to me.
The easier method is obviously having images saved in filesystem and only paths (and other image data) in the db. Then using the image method is straightforward.
I suggest you post your question also on the fpdf forum.