I’m having the same problem. Somehow Facebook doesn’t detect the first nextgen image as the primary thumbnail. Probably this has something to do with the fact that the generated image from Nextgen is too large, so the FB API jumps to the first thumbnail it finds (which is something entirely else).
I also need to dynamicly set the og:image parameter, based on the first image from a Nextgen Gallery album. I tried using jQuery, but that’s not working. If I could just do: get_nextgen_firstimage($gid); or something it would be a breeze, but no luck so far.
I’ll keep you posted Iperdesign_IT if I find a fitting solution.
Don’t know if it helps, but I’m using the following function in functions.php to retrieve the image from nextgen and place it directly in the opengraph metatag (I get the correct nextgen ID from a custom field):