Perhaps a general rule that if you are showing off pictures that you've taken of holidays, trips, fun subjects, they go in the Picture Gallery. Basically, things that most of the forum might have an interest in seeing or commenting on.
If you're showing pictures to ask for technical feedback or to illustrate the differences between two cameras/lenses, or you're asking for advice on techniques, purchase decisions etc, then it should go in the new forum. Basically, the questions that those who are interested in photography have an interest in while the subject goes over the head of others on the board?