I have a brad nailer, it is an air powered Porter Cable and I use it some for putting trim on larger projects or for holding glue joints at corners but, not on picture frames. For picture frames I just glue smaller ones at the miter and use half lap joints or splines for larger ones. I know that gluing end grain is not very strong or advisable but, usually small frames just hang on the wall and do not get a lot of handling or abuse. If I am concerned about a given frame after gluing, sometimes I drill a 1/4" hole in the edge and through the joint and insert a 1/4" dowel with glue in the hole and then trim off the excess dowel and sand smooth. The dowel joint is not very noticeable after finishing and it is on the edge which is not often seen anyway. It doesn't really matter if you dowel the miter from the side edges or the top and bottom, it is equally strong either way.
Hope that helps.
Rog