There's already the option to save images outside or inside the database. For those collecting a lot of large images, saving them outside the database may be a better choice. But I think that decision is better based on whether or not there's a need for other applications to access the images. For example, some may wish to collect and edit images.
Otherwise, I see no reason why there should be any performance issues due to the size of the database. It is, after all, a database. I suppose Firebird has more "overhead" than the file system, but I don't know why it should make any significant difference. Do you?