You're correct that it should be part of the Open Graph tags. We've heard a few reports lately that the Send button is doing it's own thing, and there are multiple reports of this to Facebook as well (not related to JFBConnect). We're investigating what they're doing, and if there's a way we can control it.
However, currently, for the Send button, the image that's chosen is out of our control.
Sorry for the inconvenience, and we'll definitely do what we can to add more control of this, if possible.
Thanks,
Alex