Wrong opengraph thumbnails when posting blog to facebook

Hi, I have encountered a very weird bug concerning some of my blog posts when publishing them to facebook,

The blog is here;

http://alianzaarkana.org/media-room/blog-latest-news/latest

for some reason some of the blog posts, for instance:
http://alianzaarkana.org/media-room/blog-latest-news/entry/community-nursery-developed-at-recover
shows the wrong thumbnails. I get to chose between three different pictures of which none are actually used in the blog in question. the same thing happens with some other posts while most of them gets the correct thumbnails when sharing, such as:

http://alianzaarkana.org/media-room/blog-latest-news/entry/los-pueblos-indigenas-de-pastaza-intensifican-la-presion-para-reclamar-sus-derechos

When looking at the code I can discern no difference in the posts that works and the ones that doesn't, Anyone else been running into this problem?