The only thing I can tell you is to ensure your using the vb4 mod to show gallery content into the forum however you can not force someone to use a program which seems to be your issue. Any gallery product out there pretty much works the same way photopost does. Your not going to see content in the forum from a product outside it.
Maybe a suggestion would be create a focal point forum on your forum that loads the gallery rss feed so it shows new posts images from the gallery and directs people there? That is kind of what I do.