I never been a fan of having default ordering for AO's, but my pleas to eliminate it have always fallen on deaf ears. But how about a compromise? Could the Pages > Image ordering combobox also offer Custom as the default?
In short, this would say, "Don't automatically order anything. If I click and drag, put them where I drag them (which it does now). And if I use the Add button, put them at the end of what's there already."