Facebook is sort of a pain to work with for images, IME. The most reliable way to throw them clues seems to be the use of their open-graph meta tags. This post on wordpress.stackexchange.com is a pretty good place to start. I'm not positive I'm looking at the right 500px portfolio for you (the link in your profile is broken, I believe), but I'm not seeing any OG tags in there. Even with the use of OG tags, though, I'm not sure there's a way to specify order beyond just ordering the tags (and I'm not sure how well that's respected by FB).
At some point in 2012, I believe 500px is supposed to offer customizable portfolio themes (most likely for "awesome" customers only), but I have a feeling your best bet may be to just wait for them to get the meta tags sorted out themselves.