I'm having the same problem.  The index.html in my gallery has this:

<meta property="og:image" content="http://photo.ntay.com/pandora2/images/P5150101-Edit.jpg" />
<meta property="og:title" content="Pandora" />
<meta property="og:type" content="website" />
<meta property="og:url" content="http://photo.ntay.com/pandora2/" />

but the Facebook popup consistently tries to use another image (P5150188-edit.jpg), which is particularly troublesome because the image is not Facebook safe.

Moreover, the Facebook post generated does not actually show the image, just a blank frame.  Looking at the properties of the image in the post on Facebook, this seems to be because it is not trying to show http://photo.ntay.com/pandora2/images/P … -Edit.jpg, but http://photo.ntay.com/images/P5150013-Edit.jpg
This is not only another completely different image (not the one that appears in the sharing popup), but it has missed the gallery name from the URL.

My config.xml has

shareURL="http://photo.ntay.com/pandora2/"

(I put that line in manually because Juicebox didn't seem to do it, but the behaviour is the same/without it.)